MIC2085
Single-Channel Low Voltage Hot Swap Controller
Features
General Description
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The MIC2085 is a single channel, positive voltage, hot
swap controller designed to allow the safe insertion of
boards into live system backplanes. The MIC2085 is
available in a 16-pin QSOP package. Using a few
external components and by controlling the gate drive
of an external N-Channel MOSFET device, the
MIC2085 provides inrush current limiting and output
voltage slew rate control in harsh, critical power supply
environments. Additionally, a circuit breaker function
will latch the output MOSFET off if the current limit
threshold is exceeded for a programmed period of time.
The device’s array of features provide a simplified yet
robust solution for many network applications in
meeting the power supply regulation requirements and
affords protection of critical downstream devices and
components.
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Pin-for-Pin Functional Equivalent to the LTC1642
2.3V to 16.5V Supply Voltage Operation
Surge Voltage Protection to 33V
Operating Temperature Range –40°C to +85°C
Active Current Regulation Limits Inrush Current
Independent of Load Capacitance
Programmable Inrush Current Limiting
Analog Foldback Current Limiting
Electronic Circuit Breaker
Dual-Level Overcurrent Fault Sensing
Fast Response to Short-Circuit Conditions ( 0V, VCC = 16.5V
Start cycle, VGATE > 0V, VCC = 2.3V
/FAULT = 0, VGATE > 1V, VCC = 16.5V
/FAULT = 0, VGATE > 1V, VCC = 2.3V
ON rising
ON falling
Specification for packaged product only.
DS20006094A-page 4
2019 Microchip Technology Inc.
MIC2085
AC ELECTRICAL CHARACTERISTICS (Note 1)
Parameters
Sym.
Min.
Typ.
Max.
Units
Fast Overcurrent Sense to
GATE Low Trip Time
tOCFAST
—
1
—
μs
VCC = 5V
VCC – VSENSE = 100 mV
CGATE = 10 nF, See Figure 1-1
Slow Overcurrent Sense
to GATE Low Trip Time
tOCSLOW
—
5
—
μs
VCC = 5V
VCC – VSENSE = 50 mV
CGATE = 0 nF, See Figure 1-1
ON Delay Filter
tONDLY
—
20
—
μs
—
FB Delay Filter
tFBDLY
—
20
—
μs
—
Note 1:
Conditions
Specification for packaged product only.
Timing Diagrams
tONDLY
VTRIPFAST
Arm Fast Comparator
48mV
(VCC – VSENSE)
Arm Slow Comparator
1.24V
ON
0
tOCSLOW
0
tOCFAST
tSTART
tPOR
1.24V
VGATE
1V
1V
CPOR
0
0
1.24V
CFILTER
GATE
0
0
FIGURE 1-1:
Current Limit Response.
1.24V
FB
0
/POR
0
FB
tPOR
0
1.24V
CPOR
0
/POR
0
FIGURE 1-2:
Power-On Reset Response.
FIGURE 1-3:
Timing.
Current Limit Threshold (mV)
1.24V
Power-On Start-Up Delay
50
20
0
200
400
600
800
1000
FB Voltage (mV)
FIGURE 1-4:
Response.
2019 Microchip Technology Inc.
Foldback Current Limit
DS20006094A-page 5
MIC2085
TEMPERATURE SPECIFICATIONS
Parameters
Sym.
Min.
Typ.
Max.
Units
Conditions
Operating Temperature Range
—
–40
—
+85
°C
—
Maximum Junction Temperature
TJ
—
—
+125
°C
—
JA
—
112
—
°C/W
—
Temperature Ranges
Package Thermal Resistances
Thermal Resistance, QSOP 16-Ld
Note 1:
The maximum allowable power dissipation is a function of ambient temperature, the maximum allowable
junction temperature and the thermal resistance from junction to air (i.e., TA, TJ, JA). Exceeding the
maximum allowable power dissipation will cause the device operating junction temperature to exceed the
maximum +125°C rating. Sustained junction temperatures above +125°C can impact the device reliability.
DS20006094A-page 6
2019 Microchip Technology Inc.
MIC2085
2.0
Note:
TYPICAL PERFORMANCE CURVES
The graphs and tables provided following this note are a statistical summary based on a limited number of
samples and are provided for informational purposes only. The performance characteristics listed herein
are not tested or guaranteed. In some graphs or tables, the data presented may be outside the specified
operating range (e.g., outside specified power supply range) and therefore outside the warranted range.
34
3.5
3.0
2.5
30
VCC = 16.5V
VCC = 5V
2.0
1.5
ITIMER (μA)
SUPPLY CURRENT (mA)
4.0
26
22
18
1.0
0.5
VCC = 2.3V
14
0.0
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-1:
Temperature.
Supply Current vs.
FIGURE 2-4:
vs. Temperature.
VCC = 16.5V
3
2
VCC = 2.3V
1
1.6
1.4
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-2:
Power-On Reset Timer
Current vs. Temperature.
VCC = 5V
FIGURE 2-5:
Overcurrent Timer (Off)
Current vs. Temperature.
30
9
8
25
VCC = 16.5V
5
4
VCC = 5V
IGATE (μA)
ICPOR (mA)
VCC = 2.3V
0
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
10
3
2
Overcurrent Timer Current
4
VCC = 16.5V
ITIMER (mA)
ICPOR (μA)
VCC = 5V
2.0
7
6
VCC = 2.3V
5
2.4
1.8
VCC = 5V
10
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
2.6
2.2
VCC = 16.5V
20
10
VCC = 2.3V
5
1
0
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-3:
Power-On Reset Timer (Off)
Current vs. Temperature.
2019 Microchip Technology Inc.
VCC = 16.5V
15
VCC = 2.3V
VCC = 5V
0
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-6:
Temperature.
Gate Pull-Up Current vs.
DS20006094A-page 7
MIC2085
25
100
90
20
IGATEOFF (mA)
80
IGATE ( A)
15
10
5
70
VCC = 16.5V
60
50
40
VCC = 5V
30
VCC = 2.3V
20
0
2
4
6
8
10
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
10 12 14 16 18
VCC (V)
FIGURE 2-7:
VCC.
Gate Pull-Up Current vs
FIGURE 2-10:
Temperature.
600
16
VCC = 5V
500
12
10
VCC = 16.5V
8
6
4
IGATEOFF (mA)
14
VGS (V)
Gate Sink Current vs.
VCC = 2.3V
400
12VCC
300
200
5VCC
100
2
0
0
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
22
20
18
16
14
12
10
8
6
4
2
0
2
External Gate Drive vs.
2
4
6
8
10
12
14
VGATE (V)
FIGURE 2-11:
Voltage.
Gate Sink Current vs. Gate
1.25
1.24
VTH (mV)
VGS (V)
FIGURE 2-8:
Temperature.
0
VCC = 16.5V
1.23
VCC = 2.3V
1.22
VCC = 5V
1.21
4
6
8
10 12 14 16 18
VCC (V)
FIGURE 2-9:
DS20006094A-page 8
External Gate Drive vs. VCC.
1.20
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-12:
POR Delay/Overcurrent
Timer Threshold vs. Temperature.
2019 Microchip Technology Inc.
MIC2085
1.30
120
VTRIPFAST (mV)
110
105
VCC = 2.3V
100
95
90 V = 5V
CC
85
VCC = 16.5V
ON THRESHOLD (V)
115
FIGURE 2-16:
vs. Temperature.
55
47
VCC = 16.5V
45
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-14:
Current Limit Threshold
(Slow Trip) vs. Temperature.
UVLO THRESHOLD (V)
VCC = 2.3V
VCC = 16.5V VCC = 5V
1.10
1.05
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-17:
vs. Temperature.
ON Pin Threshold (Falling)
40
2.5
2.4
UVLO+
2.2
2.1
2.0
UVLO–
1.8
1.7
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-15:
Temperature.
ON THRESHOLD (V)
VCC = 5V
1.9
ON Pin Threshold (Rising)
1.15
UVLO Threshold vs.
2019 Microchip Technology Inc.
ON PIN INPUT CURRENT (nA)
VTRIPSLOW (mV)
VCC = 2.3V
49
2.3
VCC = 2.3V
1.20
53
51
VCC = 5V
1.20
1.15
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
80
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-13:
Current Limit Threshold
(Fast Trip) vs. Temperature.
VCC = 16.5V
1.25
35
30
25
20
VCC = 16.5V
15
10
VCC = 2.3V
VCC = 5V
5
0
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-18:
Temperature.
ON Input Current vs.
DS20006094A-page 9
COMPARATOR OFFSET VOLTAGE (V)
MIC2085
FB THRESHOLD (V)
1.30
VCC = 16.5V
1.25
1.20
VCC = 2.3V
VCC = 5V
1.15
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FB (Power Good) Threshold
0.4
0.3
VCC = 5V
0.2
VCC = 16.5V
0.1
VCC = 2.3V
0.0
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
FIGURE 2-22:
vs. Temperature.
Comparator Offset Voltage
VCC = 16.5V
1.25
VCC = 2.3V
V CC
CPOR ON
1V/div 1V/div 5V/div
1.30
1.20
1.15
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
VIN = 12V
RLOAD = 4.8Ω
CLOAD = 1000 μF
IIN =IOUT
1A/div
OVERVOLTAGE PIN THRESHOLD (V)
FIGURE 2-19:
vs. Temperature.
0.5
TIME (20ms/div.)
FIGURE 2-20:
vs. Temperature.
Overvoltage Pin Threshold
FIGURE 2-23:
12V Hot Insert Response.
VCC = 2.3V
14
10
-40 -20 0 20 40 60 80 100
TEMPERATURE (°C)
VIN = 12V
RLOAD = 4.8Ω
CLOAD = 1000 μF
VOUT
5V/div
18
VCC = 5V
CPOR
1V/div
VCC = 16.5V
/POR
10V/div
IPULLUP (μA)
22
ON
1V/div
26
TIME (20ms/div.)
FIGURE 2-21:
Output Signal Pull-Up
Current vs. Temperature.
DS20006094A-page 10
FIGURE 2-24:
12V Turn-On Response.
2019 Microchip Technology Inc.
VIN = 12V
RLOAD = 0
CLOAD = 1000 μF
I IN
1A/div
VOUT
5V/div
VIN = 12V
RLOAD = 3.4Ω
CLOAD = 5700 μF
/FAULT
10V/div
VOUT /FAULT VCC
10V/div 10V/div 5V/div
CFILTER ON
1V/div 1V/div
MIC2085
TIME (10ms/div.)
Inrush Current Response.
FIGURE 2-28:
Turn-On into Short-Circuit.
ON
IIN = IOUT GATE
20V/div 1V/div
2A/div
FIGURE 2-25:
TIME (10ms/div.)
VOUT
5V/div
VIN = 12V
RLOAD = 4.8Ω
CLOAD = 1000 μF
SW2 = HIGH
TIME (2.5ms/div.)
Turn Off: Normal Discharge.
ON
IIN = IOUT GATE
2A/div 20V/div 1V/div
FIGURE 2-26:
VOUT
5V/div
VIN = 12V
RLOAD = 4.8Ω
CLOAD = 1000 μF
SW2 = LOW
TIME (2.5ms/div.)
FIGURE 2-27:
Discharge.
Turn Off: Crowbar
2019 Microchip Technology Inc.
DS20006094A-page 11
MIC2085
RSENSE
5%
IIN
VIN
12V
1
2
3
C4
0.47μF
Q1
Si7892DP
(PowerPAK™ SO-8)
VOUT
4
RLOAD
R1
19,20
R2
N
1%
SW1
ON/OFF
VCC
18
SENSE
GATE
4
IOUT
CLOAD
R4
N
1%
17
C2
0.022μF
ON
FB
R3
N
1%
8
R5
N
1%
MIC2085
/POR
CPOR
3
C3
0.047μF
GND
9,10
5
Downstream
Signal
CFILTER
2
C4
0.047μF
For clarity, not all pins are shown.
FIGURE 2-29:
DS20006094A-page 12
Test Circuit.
2019 Microchip Technology Inc.
MIC2085
3.0
PIN DESCRIPTIONS
The descriptions of the pins are listed in Table 3-1.
TABLE 3-1:
Pin Number
MIC2085
PIN FUNCTION TABLE
Pin Name
Description
CRWBR
Overvoltage Timer and Crowbar Circuit Trigger: A capacitor connected to this pin
sets the timer duration for which an overvoltage condition will trigger an external
crowbar circuit. This timer begins when the OV input rises above its threshold as an
internal 45 μA current source charges the capacitor. Once the voltage reaches
470 mV, the current increases to 1.5 mA.
CFILTER
Current Limit Response Timer: A capacitor connected to this pin defines the period
of time (tOCSLOW) in which an overcurrent event must last to signal a fault condition
and trip the circuit breaker. If no capacitor is connected, then tOCSLOW defaults to
5 μs.
CPOR
Power-On Reset Timer: A capacitor connected between this pin and ground sets
the start-up delay (tSTART) and the power-on reset interval (tPOR). When VCC rises
above the UVLO threshold, the capacitor connected to CPOR begins to charge.
When the voltage at CPOR crosses 1.24V, the start-up threshold (VSTART), a start
cycle is initiated if ON is asserted while capacitor CPOR is immediately discharged
to ground. When the voltage at FB rises above VFB, capacitor CPOR begins to
charge again. When the voltage at CPOR rises above the power-on reset delay
threshold (VTH), the timer resets by pulling CPOR to ground, and /POR is deasserted. If CPOR = 0, then tSTART defaults to 20 μs.
ON
ON Input: Active-high. The ON pin, an input to a Schmitt-triggered comparator used
to enable/disable the controller, is compared to a VTH reference with 100 mV of
hysteresis. Once a logic high is applied to the ON pin (VON > 1.24V), a start-up
sequence is initiated as the GATE pin starts ramping up towards its final operating
voltage. When the ON pin receives a low logic signal (VON < 1.14V), the GATE pin
is grounded and /FAULT is high if VCC is above the UVLO threshold. ON must be
low for at least 20 μs in order to initiate a start-up sequence. Additionally, toggling
the ON pin LOW to HIGH resets the circuit breaker.
/POR
Power-On Reset Output: Open-drain N-Channel device, active-low. This pin
remains asserted during start-up until a time period tPOR after the FB pin voltage
rises above the power good threshold (VFB). The timing capacitor CPOR determines tPOR. When an output undervoltage condition is detected at the FB pin, /POR
is asserted for a minimum of one timing cycle, tPOR. The /POR pin has a weak
pull-up to VCC.
/FAULT
Circuit Breaker Fault Status Output: Open-drain N-Channel device, active-low. The
/FAULT pin is asserted when the circuit breaker trips due to an overcurrent condition. Also, this pin indicates undervoltage lockout and overvoltage fault conditions.
The /FAULT pin has a weak pull-up to VCC.
7
FB
Power Good Threshold Input: This input is internally compared to a 1.24V reference with 3 mV of hysteresis. An external resistive divider may be used to set the
voltage at this pin. If this input momentarily goes below 1.24V, then /POR is activated for one timing cycle, tPOR, indicating an output undervoltage condition. The
/POR signal deasserts one timing cycle after the FB pin exceeds the power good
threshold by 3 mV. A 5 μs filter on this pin prevents glitches from inadvertently activating this signal.
8
GND
1
2
3
4
5
6
9
OV
2019 Microchip Technology Inc.
Ground Connection: Tie to analog ground
OV Input: When the voltage on OV exceeds its trip threshold, the GATE pin is
pulled low and the CRWBR timer starts. If OV remains above its threshold long
enough for CRWBR to reach its trip threshold, the circuit breaker is tripped. Otherwise, the GATE pin begins to ramp up one POR timing cycle after OV drops below
its trip threshold.
DS20006094A-page 13
MIC2085
TABLE 3-1:
PIN FUNCTION TABLE (CONTINUED)
Pin Number
MIC2085
Pin Name
10
COMPOUT
11
COMP+
Comparator’s Non-Inverting Input.
12
COMP–
Comparator’s Inverting Input.
13
REF
Reference Output: 1.24V nominal. Tie a 0.1 μF capacitor to ground to ensure stability.
GATE
Gate Drive Output: Connects to the gate of an external N-Channel MOSFET. An
internal clamp ensures that no more than 13V is applied between the GATE pin and
the source of the external MOSFET. The GATE pin is immediately brought low
when either the circuit breaker trips or an undervoltage lockout condition occurs.
SENSE
Circuit Breaker Sense Input: A resistor between this pin and VCC sets the current
limit threshold. Whenever the voltage across the sense resistor exceeds the slow
trip current limit threshold (VTRIPSLOW), the GATE voltage is adjusted to ensure a
constant load current. If VTRIPSLOW (48 mV) is exceeded for longer than time period
tOCSLOW, then the circuit breaker is tripped and the GATE pin is immediately pulled
low. If the voltage across the sense resistor exceeds the fast trip circuit breaker
threshold, VTRIPFAST, at any point due to fast, high amplitude power supply faults,
then the GATE pin is immediately brought low without delay. To disable the circuit
breaker, the SENSE and VCC pins can be tied together.
The default VTRIPFAST for either device is 95 mV. Other fast trip thresholds are
available: 150 mV, 200 mV, or OFF (VTRIPFAST disabled). Please contact Microchip
for availability of other options.
VCC
Positive Supply Input: 2.3V to 16.5V. The GATE pin is held low by an internal
undervoltage lockout circuit until VCC exceeds a threshold of 2.18V.If VCC exceeds
16.5V, an internal shunt regulator protects the chip from VCC and SENSE pin voltages up to 33V.
14
15
16
DS20006094A-page 14
Description
Uncommitted Comparator’s Open-Drain Output.
2019 Microchip Technology Inc.
MIC2085
4.0
FUNCTIONAL DESCRIPTION
4.1
Hot Swap Insertion
When circuit boards are inserted into live system
backplanes and supply voltages, high inrush currents
can result due to the charging of bulk capacitance that
resides across the supply pins of the circuit board. This
inrush current, although transient in nature, may be
high enough to cause permanent damage to on-board
components or may cause the system’s supply
voltages to go out of regulation during the transient
period which may result in system failures. The
MIC2085 acts as a controller for external N-Channel
MOSFET devices in which the gate drive is controlled
to provide inrush current limiting and output voltage
slew rate control during hot plug insertions.
4.2
Power Supply
VCC is the supply input to the MIC2085 controller with
a voltage range of 2.3V to 16.5V. The VCC input can
withstand transient spikes up to 33V. In order to help
suppress transients and ensure stability of the supply
voltage, a capacitor of 1.0 μF to 10 μF from VCC to
ground is recommended. Alternatively, a low pass filter,
shown in the typical application circuit, can be used to
eliminate high frequency oscillations as well as help
suppress transient spikes.
4.3
Start-Up Cycle
When the voltage on the ON pin rises above its
threshold of 1.24V, the MIC2085/6 first checks that its
supply (VCC) is above the UVLO threshold. If it does
check above, the device is enabled and an internal
2 μA current source begins charging capacitor CPOR
to 1.24V to initiate a start-up sequence (i.e., start-up
delay times out). Once the start-up delay (tSTART)
elapses, CPOR is pulled immediately to ground and a
15 μA current source begins charging the GATE output
to drive the external MOSFET that switches VIN to
VOUT. The programmed start-up delay is calculated
using the following equation:
EQUATION 4-1:
V TH
t START = C POR --------------- 0.62 C POR F
I CPOR
Where:
VTH = 1.24V, the POR delay threshold.
ICPOR = 2 μA, the POR timer current.
2019 Microchip Technology Inc.
As the GATE voltage continues ramping toward its final
value (VCC + VGS) at a defined slew rate (See the Load
Capacitance Dominated Start-Up/Gate Capacitance
Dominated Start-Up sections), a second CPOR timing
cycle begins if:
1.
2.
/FAULT is high and
CFILTER is low (i.e., not an overvoltage, undervoltage lockout, or overcurrent state).
This second timing cycle, tPOR, starts when the voltage
at the FB pin exceeds its threshold (VFB) indicating that
the output voltage is valid. The time period tPOR is
equivalent to tSTART and sets the interval for the /POR
to go Low-to-High after power is good (See Figure 1-2).
Active current regulation is employed to limit the inrush
current transient response during start-up by regulating
the load current at the programmed current limit value
See the Current Limiting and Dual-Level Circuit
Breaker section. The following equation is used to
determine the nominal current limit value:
EQUATION 4-2:
V TRIPSLOW
48mV
I LIM = ---------------------------- = ------------------R SENSE
R SENSE
Where:
VTRIPSLOW = The current limit slow trip threshold
found in Electrical Characteristics.
RSENSE = The selected value that will set the desired
current limit.
There are two basic start-up modes for the MIC2085/6:
• Start-up dominated by load capacitance.
• Start-up dominated by total gate capacitance.
The magnitude of the inrush current delivered to the
load will determine the dominant mode. If the inrush
current is greater than the programmed current limit
(ILIM), then load capacitance is dominant. Otherwise,
gate capacitance is dominant. The expected inrush
current may be calculated using the following equation:
EQUATION 4-3:
C LOAD
C LOAD
INRUSH I GATE ---------------- 15A ----------------C GATE
C GATE
Where:
IGATE = The pin pull-up current.
CLOAD = The load capacitance.
CGATE = The total GATE capacitance (CISS of the
external MOSFET and any external capacitor
connected from the MIC2085/6 GATE pin to ground).
DS20006094A-page 15
MIC2085
4.3.1
LOAD CAPACITANCE DOMINATED
START-UP
In this case, the load capacitance, CLOAD, is large
enough to cause the inrush current to exceed the
programmed current limit, but is less than the fast-trip
threshold (or the fast-trip threshold is disabled, ‘M’
option). During start-up under this condition, the load
current is regulated at the programmed current limit
value (ILIM) and held constant until the output voltage
rises to its final value. The output slew rate and
equivalent GATE voltage slew rate is computed by the
following equation:
EQUATION 4-4:
EQUATION 4-6:
I GATE
C GATE MIN = -------------- C LOAD
I LIM
Where:
CGATE = The sum of the MOSFET input capacitance
(CISS) and the value of the external capacitor
connected to the GATE pin of the MOSFET.
Once CGATE is determined, use the following equation
to determine the output slew rate for gate capacitance
dominated start-up.
EQUATION 4-7:
I LIM
d V OUT dt = ----------------C LOAD
I GATE
dV OUT dt OUTPUT = ---------------C GATE
Where:
ILIM = The programmed current limit value.
Consequently, the value of CFILTER must be selected
to ensure that the overcurrent response time, tOCSLOW,
exceeds the time needed for the output to reach its final
value. For example, given a MOSFET with an input
capacitance CISS = CGATE = 4700 pF, CLOAD is
2200 μF, and ILIM is set to 6A with a 12V input, then the
load capacitance dominates as determined by the
calculated INRUSH > ILIM. Therefore, the output
voltage slew rate determined from Equation 4-4 is:
Table 4-1 depicts the output slew rate for various
values of CGATE.
TABLE 4-1:
EQUATION 4-5:
6A
dV OUT dt = -------------------- = 2.73V /ms
2200F
4.3.2
GATE CAPACITANCE DOMINATED
START-UP
In this case, the value of the load capacitance relative
to the GATE capacitance is small enough such that the
load current during start-up never exceeds the current
limit threshold as determined by Equation 4-3. The
minimum value of CGATE that will ensure that the
current limit is never exceeded is given by the equation
below:
DS20006094A-page 16
CGATE
Output Slew Rate
0.001 μF
15 V/ms
0.01 μF
1.5 V/ms
0.1 μF
0.150 V/ms
1 μF
Note 1: IGATE = 15 μA.
4.4
The resulting tOCSLOW needed to achieve a 12V output
is approximately 4.5 ms. See the Power-On Reset,
Start-Up, and Overcurrent Timer Delays section to
calculate tOCSLOW.
OUTPUT SLEW RATE
SELECTION FOR GATE
CAPACITANCE DOMINATED
START-UP
0.015 V/ms
Current Limiting and Dual-Level
Circuit Breaker
Many applications will require that the inrush and
steady state supply current be limited at a specific
value in order to protect critical components within the
system. Connecting a sense resistor between the VCC
and SENSE pins sets the nominal current limit value of
the MIC2085 and the current limit is calculated using
Equation 4-2. However, the MIC2085 exhibits foldback
current limit response. The foldback feature allows the
nominal current limit threshold to vary from 24 mV up to
48 mV as the FB pin voltage increases or decreases.
When FB is at 0V, the current limit threshold is 24 mV
and for FB ≥ 0.6V, the current limit threshold is the
nominal 48 mV (see Figure 1-4 for Foldback Current
Limit Response characteristic). The MIC2085 also
features a dual-level circuit breaker triggered via 48 mV
and 95 mV current limit thresholds sensed across the
VCC and SENSE pins.
2019 Microchip Technology Inc.
MIC2085
The first level of the circuit breaker functions as follows.
Once the voltage sensed across these two pins
exceeds 48 mV, the overcurrent timer, its duration set
by capacitor CFILTER, starts to ramp the voltage at
CFILTER using a 2 μA constant current source. If the
voltage at CFILTER reaches the overcurrent timer
threshold (VTH) of 1.24V, then CFILTER immediately
returns to ground as the circuit breaker trips and the
GATE output is immediately shut down.
For the second level, if the voltage sensed across VCC
and SENSE exceeds 95 mV at any time, the circuit
breaker trips and the GATE shuts down immediately,
bypassing the overcurrent timer period. To disable
current limit and circuit breaker operation, tie the
SENSE and VCC pins together and the CFILTER pin to
ground.
4.5
Output Undervoltage Detection
The MIC2085 employs output undervoltage detection
by monitoring the output voltage through a resistive
divider connected at the FB pin. During turn-on, while
the voltage at the FB pin is below the threshold (VFB),
the /POR pin is asserted low. Once the FB pin voltage
crosses VFB, a 2 μA current source charges capacitor
CPOR. Once the CPOR pin voltage reaches 1.24V, the
time period tPOR elapses as the CPOR pin is pulled to
ground and the /POR pin goes high. If the voltage at FB
drops below VFB for more than 10 μs, the /POR pin
resets for at least one timing cycle defined by tPOR (see
Application Information for an example).
4.6
A capacitor connected to CFILTER is used to set the
timer that activates the circuit breaker during
overcurrent conditions. When the voltage across the
sense resistor exceeds the slow trip current limit
threshold of 48 mV, the overcurrent timer begins to
charge for a period, tOCSLOW, determined by CFILTER.
If no capacitor is used at CFILTER, then tOCSLOW
defaults to 5 μs. If tOCSLOW elapses, then the circuit
breaker is activated and the GATE output is
immediately pulled to ground. The following equation is
used to determine the overcurrent timer period,
tOCSLOW.
EQUATION 4-8:
V TH
t OCSLOW = C FILTER ----------------- 0.062 C FILTER F
I TIMER
Where:
VTH = The CFILTER timer threshold: 1.24V.
ITIMER = The overcurrent timer current: 20 μA.
Table 4-2 and Table 4-3 provide a quick reference for
several timer calculations using select standard value
capacitors.
TABLE 4-2:
Input Overvoltage Protection
The MIC2085 monitors and detects overvoltage
conditions in the event of excessive supply transients
at the input. Whenever the overvoltage threshold (VOV)
is exceeded at the OV pin, the GATE is pulled low and
the output is shut off. The GATE will begin ramping one
POR timing cycle after the OV pin voltage drops below
its threshold. An external CRWBR circuit, as shown in
the Typical Application Circuit, provides a time period
that an overvoltage condition must exceed in order to
trip the circuit breaker. When the OV pin exceeds the
overvoltage threshold (VOV), the CRWBR timer begins
charging the CRWBR capacitor initially with a 45 μA
current source.Once the voltage at CRWBR exceeds
its threshold (VCR) of 0.47V, the CRWBR current
immediately increases to 1.5 mA and the circuit
breaker is tripped, necessitating a device reset by
toggling the ON pin from low to high.
4.7
connected to CPOR sets the interval, tPOR, and tPOR is
equivalent to the start-up delay, tSTART (see
Equation 4-1).
Power-On Reset, Start-Up, and
Overcurrent Timer Delays
The Power-On Reset delay, tPOR, is the time period for
the /POR pin to go high once the voltage at the FB pin
exceeds the power good threshold (VTH). A capacitor
2019 Microchip Technology Inc.
CPOR
SELECTED POWER-ON
RESET AND START-UP
DELAYS
tPOR = tSTART
0.01 μF
6 ms
0.02 μF
12 ms
0.033 μF
18.5 ms
0.05 μF
30 ms
0.1 μF
60 ms
0.33 μF
200 ms
TABLE 4-3:
SELECTED OVERCURRENT
TIMER DELAYS
CFILTER
tOCSLOW
1800 pF
100 μs
4700 pF
290 μs
8200 pF
500 μs
0.01 μF
620 μs
0.02 μF
1.2 ms
0.033 μF
2.0 ms
0.05 μF
3.0 ms
0.1 μF
6.2 ms
0.33 μF
20.7 ms
DS20006094A-page 17
MIC2085
5.0
APPLICATION INFORMATION
5.1
Output Undervoltage Detection
For output undervoltage detection, the first
consideration is to establish the output voltage level
that indicates “power is good.” For this example, the
output value for which a 12V supply will signal “good” is
11V. Next, consider the tolerances of the input supply
and FB threshold (VFB). For this example, the 12V
supply varies ±5%, thus the resulting output voltage
may be as low as 11.4V and as high as 12.6V.
Additionally, the FB threshold has ±50 mV tolerance
and may be as low as 1.19V and as high as 1.29V.
Thus, to determine the values of the resistive divider
network (R5 and R6) at the FB pin, shown in
Figure 5-1, use the following iterative design
procedure.
1.
Choose R6 so as to limit the current through the
divider to approximately 100 μA or less.
Substituting these values into Equation 5-3 now yields
R5 = 100.11 kΩ. A standard 100 kΩ ±1% is selected.
Now, consider the 11.4V minimum output voltage, the
lower tolerance for R6 and higher tolerance for R5,
13.17 kΩ and 101 kΩ, respectively. With only 11.4V
available, the voltage sensed at the FB pin exceeds
VFB(MAX), thus the /POR signal will transition from low
to high, indicating “power is good” given the worse case
tolerances of this example.
5.2
The external CRWBR circuit shown in Figure 5-1
consists of capacitor C4, resistor R7, NPN transistor
Q2, and SCR Q3. The capacitor establishes a time
duration for an overvoltage condition to last before the
circuit breaker trips. The CRWBR timer duration is
approximated by the following equation:
EQUATION 5-4:
EQUATION 5-1:
V FB MAX 1.29V
R6 ------------------------ ---------------- 12.9k
100A
100A
R6 is chosen as 13.3 kΩ ±1%.
2.
Next, determine R5 using the output “good” voltage of 11V and the following equation:
EQUATION 5-2:
R5 + R6
V OUT GOOD = V FB -------------------R6
Using some basic algebra and simplifying Equation 5-2
to isolate R5, yields:
Input Overvoltage Protection
C4 V CR
t OVCR ----------------------- 0.01 C4 F
I CR
Where:
VCR = The CRWBR pin threshold: 0.47V.
ICR = The CRWBR pin current during the timer
period: 45 μA.
See the CRWBR timer pin description in Table 3-1 for
more information. A design approach similar to the
previous undervoltage detection example is
recommended for the overvoltage protection circuitry,
resistors R2 and R3 in Figure 5-1. For input
overvoltage protection, the first consideration is to
establish the input voltage level that indicates an
overvoltage triggering a system (output voltage)
shutdown. For this example, the input value for which a
12V supply will signal an “output shutdown” is 13.2V
(+10%). Similarly, from the previous example:
1.
Choose R3 to satisfy 100 μA condition.
EQUATION 5-5:
EQUATION 5-3:
V OV MIN 1.19V
R3 ------------------------ ---------------- 11.9k
100A
100A
V OUT GOOD
R5 = R6 --------------------------------–1
V FB MAX
Where:
VFB(MAX) = 1.29V.
VOUT(GOOD) = 11V.
R6 = 13.3 kΩ.
DS20006094A-page 18
R3 is chosen as 13.7 kΩ ±1%.
2.
Thus, following the previous example and substituting R2 and R3 for R5 and R6, respectively,
and 13.2V overvoltage for 11V output “good”,
the same formula yields R2 of 138.3 kΩ. The
next highest standard 1% value is 140 kΩ.
2019 Microchip Technology Inc.
MIC2085
Now, consider the 12.6V maximum input voltage (VCC
+5%), the higher tolerance for R3 and lower tolerance
for R2, 13.84 kΩ and 138.60 kΩ, respectively. With a
12.6V input, the voltage sensed at the OV pin is below
VOV(MIN), and the MIC2085 will not indicate an
overvoltage condition until VCC exceeds at least 13.2V.
Q1
IRF7822
(SO-8)
RSENSE
0.012Ω
1 2% 2
VIN
12V
3
R2
140kΩ
1%
R1
100kΩ
VOUT
12V@3A
4
CLOAD
220μF
C1
1μF
16
VCC
R4
10Ω
15
SENSE
14
GATE
4
C2
0.022μF
ON
FB
7
MIC2085
9
R6
13.3kΩ
1%
/POR
OV
/FAULT
R3
13.7kΩ
1%
R5
100kΩ
1%
CPOR
GND
3
CRWBR
8
C3
0.05μF
5
6
Downstream
Signals
Q2
2N4401
1
C4
0.01μF
C5
0.033μF
Q3
TCR22-4
*R7
180Ω
Overvoltage (Input) = 13.3V
Undervoltage (Output) = 11.0V
POR/START-UP Delay = 30ms
*R7 needed when using a sensitive gate SCR.
Additional pins omitted for clarity.
FIGURE 5-1:
5.3
Undervoltage/Overvoltage Circuit.
PCB Connection Sense
There are several configuration options for the
MIC2085’s ON pin to detect if the PCB has been fully
seated in the backplane before initiating a start-up
cycle. In the Typical Application Circuit, the MIC2085 is
mounted on the PCB with a resistive divider network
connected to the ON pin. R2 is connected to a short pin
on the PCB edge connector. Until the connectors mate,
the ON pin is held low, which keeps the GATE output
charge pump off. Once the connectors mate, the
resistor network is pulled up to the input supply, 12V in
this example, and the ON pin voltage exceeds its
threshold (VON) of 1.24V and the MIC2085 initiates a
start-up cycle.
In Figure 5-2, the connection sense, consisting of a
logic-level discrete MOSFET and a few resistors,
allows for interrupt control from the processor or other
signal controller to shut off the output of the MIC2085.
R4 keeps the GATE of Q2 at VIN until the connectors
are fully mated. A logic low at the /ON_OFF signal turns
Q2 off and allows the ON pin to pull up above its
threshold and initiate a start-up cycle. Applying a logic
high at the /ON_OFF signal will turn Q2 on and short
the ON pin of the MIC2085 to ground, which turns off
the GATE output charge pump.
2019 Microchip Technology Inc.
DS20006094A-page 19
MIC2085
Backplane PCB Edge
Connector Connector
VIN
12V
RSENSE
0.008Ω
1 2% 2
Long
Pin
3
C1
1μF
Q1
Si7860DP
(PowerPAKTM SO-8)
CLOAD
220μF
R5
10Ω
Short
Pin
16
R4
10kΩ
4
SENSE
GATE
14
ON
C2
0.01μF
R1
20kΩ
/ON_OFF
R6
127kΩ
1%
15
VCC
R2
20kΩ
R3
100Ω
VOUT
12V@5A
4
FB
7
R7
16.2kΩ
1%
MIC2085
*Q2
/POR
/FAULT
PCB Connection Sense
CPOR
3
Long
Pin
5.4
1
Downstream
Signals
GND
8
C2
0.05μF
GND
FIGURE 5-2:
5
Undervoltage (Output) = 11.4V
POR/START-UP DELAY = 30ms
*Q2 is TN0201T (SOT-23)
Additional pins omitted for clarity.
PCB Connection Sense with ON/OFF Control.
Higher UVLO Setting
Once a PCB is inserted into a backplane (power
supply), the internal UVLO circuit of the MIC2085 holds
the GATE output charge pump off until VCC exceeds
2.18V. If VCC falls below 2V, the UVLO circuit pulls the
GATE output to ground and clears the overvoltage
and/or current limit faults. For a higher UVLO threshold,
the circuit in Figure 5-3 can be used to delay the output
MOSFET from switching on until the desired input
voltage is achieved. The circuit allows the charge pump
to remain off until VIN exceeds (1 + R1/R2) x 1.24V.
The GATE drive output will be shut down when VIN falls
below (1 + R1/R2) x 1.14V. In the example circuit
(Figure 5-3), the rising UVLO threshold is set at
approximately 11V and the falling UVLO threshold is
established as 10.1V. The circuit consists of an external
resistor divider at the ON pin that keeps the GATE
output charge pump off until the voltage at the ON pin
exceeds its threshold (VON) and after the start-up time
relapses.
DS20006094A-page 20
2019 Microchip Technology Inc.
MIC2085
Q1
IRF7822
(SO-8)
RSENSE
0.010Ω
1 2% 2
VIN
12V
3
VOUT
12V@4A
4
CLOAD
220μF
C1
1μF
R1
392kΩ
1%
16
VCC
R3
10Ω
15
SENSE
GATE
4
R4
127kΩ
1%
14
C2
0.01μF
ON
MIC2085
R2
49.9kΩ
1%
FB
R5
16.2kΩ
1%
/POR
CPOR
3
GND
7
5
Downstream
Signal
8
C3
0.1μF
Undervoltage Lockout (Rising) = 11.0V
Undervoltage Lockout (Falling) = 10.1V
Undervoltage (Output) = 11.4V
POR/START-UP Delay = 60ms
Additional pins omitted for clarity.
FIGURE 5-3:
5.5
Higher UVLO Setting.
Auto-Retry Upon Overcurrent
Faults
The MIC2085 can be configured for automatic restart
after a fault condition. Placing a diode between the ON
and/FAULT pins, as shown in Figure 5-4, will enable
the auto-restart capability of the controller. When an
application is configured for auto-retry, the overcurrent
timer should be set to minimize the duty cycle of the
overcurrent response to prevent thermal runaway of
the power MOSFET. See the MOSFET Transient
Thermal Issues section for further detail. A limited duty
cycle is achieved when the overcurrent timer duration
(tOCSLOW) is much less than the start-up delay timer
duration (tSTART) and is calculated using the following
equation:
EQUATION 5-6:
t OCSLOW
Auto – Retry Duty Cycle = ---------------------- 100%
t START
2019 Microchip Technology Inc.
DS20006094A-page 21
MIC2085
Q1
IRF7822
(SO-8)
RSENSE
0.012Ω
1 5% 2
VIN
5V
3
VOUT
5V@2.5A
4
CLOAD
220μF
C1
1μF
R1
47kΩ
16
VCC
R3
10Ω
15
SENSE
GATE
R2
33kΩ
ON SIGNAL
4
ON
FB
MIC2085
6
/FAULT
CPOR
GND
3
C3
0.02μF
Undervoltage (Output) = 4.27V
POR/START-UP Delay = 12ms
Circuit Breaker Response Time = 290μs
Auto-Retry Duty Cycle = 2.5%
Additional pins omitted for clarity.
5.6
7
R5
14.7kΩ
1%
/POR
FIGURE 5-4:
14
C2
0.022μF
D1
1N914
/FAULT
OUTPUT
R4
34kΩ
1%
8
5
CFILTER
Downstream
Signal
2
C4
4700pF
Auto-Retry Configuration.
An InfiniBand™ Application
Circuit
The circuit in Figure 5-5 depicts a single 50W
InfiniBand™ module using the MIC2085 controller. An
InfiniBand™ backplane distributes bulk power to
multiple plug-in modules that employ DC/DC
converters for local supply requirements. The circuit in
Figure 5-5 distributes 12V from the backplane to the
MIC2182 DC/DC converter that steps down +12V to
+3.3V for local bias. The pass transistor, Q1, isolates
the MIC2182’s input capacitance during module plug-in
and allows the backplane to accommodate additional
plug-in modules without affecting the other modules on
the backplane. The two control input signals are
VBxEn_L (active-low) and a Local Power Enable
(active-high). The MIC2085 in the circuit of Figure 5-5
performs a number of functions. The gate output of Q1
is enabled by the two bit input signal VBxEn_L, Local
Power Enable = [0,1]. Also, the MIC2085 limits the
drain current of Q1 to 7A, monitors VB_In for an
overvoltage condition greater than 16V, and enables
the MIC2182 DC/DC converter downstream to supply
a local voltage rail. The uncommitted comparator is
used to monitor VB_In for an undervoltage condition of
less than 10V, indicated by a logic-low at the
comparator output (COMPOUT). COMPOUT may be
used to control a downstream device such as another
DC/DC converter. Additionally, the MIC2085 is
configured for auto-retry upon an overcurrent fault
condition by placing a diode (D1) between the /FAULT
and ON pins of the controller.
DS20006094A-page 22
2019 Microchip Technology Inc.
MIC2085
InfiniBandTM
Backplane
InfiniBandTM
MODULE
MIC2182
DC/DC Converter
Q1
IRF7822
(SO-8)
RSENSE
ȍ
1 5% 2
Long
VB_In
(12V)
3
R2
Nȍ
1%
Short
VBxEn_L
R3
Nȍ
1%
R4
Nȍ
1%
VIN
4
16
R6
ȍ
15
VCC
SENSE
9
OV
11
COMP+
GATE 14
R7
Nȍ
1%
C2
0.022μF
/UV
COMPOUT 10
R5
Nȍ
1%
3
12
C1
0.01μF
C5
0.033μF
Long
Local Power
Enable
FIGURE 5-5:
ON
R1
Nȍ
A 50W InfiniBand™ Application.
The MIC2085 uses a low-value sense resistor to
measure the current flowing through the MOSFET
switch (and therefore the load). This sense resistor is
nominally
valued
at
48 mV/ILOAD(CONT). To
accommodate worst-case tolerances for both the
sense resistor (allow ±3% over time and temperature
for a resistor with ±1% initial tolerance) and still supply
the maximum required steady-state load current, a
slightly more detailed calculation must be used. The
current limit threshold voltage (the “trip point”) for the
MIC2085 may be as low as 40 mV, which would equate
to a sense resistor value of 40 mV/ILOAD(CONT).
Carrying the numbers through for the case where the
value of the sense resistor is 3% high yields:
EQUATION 5-7:
40mV
38.8mV
= -------------------------------------------------- = ---------------------------------1.03 I LOAD CONT
I LOAD CONT
Once the value of RSENSE has been chosen in this
manner, it is good practice to check the maximum
ILOAD(CONT) that the circuit may let through in the case
of tolerance build-up in the opposite direction. Here, the
2019 Microchip Technology Inc.
D1
1N914
CFILTER GND
Sense Resistor Selection
R SENSE MAX
/FAULT 6
REF
2
RUN/SS
FB 7
COMP–
C3
0.1μF
Power-On Reset
Output
MIC2085
C4
0.022μF
VB_Ret
5.7
13
CPOR
/POR
5
8
R8
Nȍ
1%
3.3V @ 4A
4
CRWBR
1
GND
Overvoltage (Input) = 16.0V
Undervoltage (Input) = 10.0V
Undervoltage (Output) &
Power Good (Output) = 10.0V
Circuit Breaker Response Time = 1.2ms
POR/START-UP Delay = 18.5ms
Auto-Retry Duty Cycle = 6.5%
worst-case maximum current is found using a 55 mV
trip voltage and a sense resistor that is 3% low in value.
The resulting equation is:
EQUATION 5-8:
I LOAD CONTMAX =
55mV
56.7mV --------------------------------------------------- = ---------------------------------0.97 R SENSE NOM
R SENSE NOM
As an example, if an output must carry a continuous 6A
without nuisance trips occurring, Equation 5-7 yields
the following:
EQUATION 5-9:
38.8mV
R SENSE MAX = ------------------- = 6.5m
6A
The next lowest standard value is 6.0 mW. At the other
set of tolerance extremes for the output in question:
DS20006094A-page 23
MIC2085
EQUATION 5-10:
56.7mV
I LOAD CONTMAX = ------------------- = 9.45 A
6.0m
The result is almost 10A. Knowing this final datum, we
can determine the necessary wattage of the sense
resistor, using P = I2R, where I will be ILOAD(CONT, MAX),
and R will be 0.97 x RSENSE(NOM). These numbers
yield the following:
EQUATION 5-11:
2
P MAX = 10 A 5.82m = 0.582W
In this example, a 1W sense resistor is sufficient.
5.8
MOSFET Selection
Selecting the proper external MOSFET for use with the
MIC2085 involves three straightforward tasks:
1.
2.
3.
5.9
Choice of a MOSFET that meets minimum voltage requirements.
Selection of a device to handle the maximum
continuous current (steady-state thermal
issues).
Verify the selected part’s ability to withstand any
peak currents (transient thermal issues).
MOSFET Voltage Requirements
The first voltage requirement for the MOSFET is that
the drain-source breakdown voltage of the MOSFET
must be greater than VIN(MAX). For instance, a 16V
input may reasonably be expected to see
high-frequency transients as high as 24V. Therefore,
the drain-source breakdown voltage of the MOSFET
must be at least 25V. For ample safety margin and
standard availability, the closest minimum value should
be 30V.
The second breakdown voltage criterion that must be
met is a bit subtler than simple drain-source breakdown
voltage. The gate of the external MOSFET is driven up
to a maximum of 21V by the internal output MOSFET.
At the same time, if the output of the external MOSFET
(its source) is suddenly subjected to a short, the
gate-source voltage will go to (21V – 0V) = 21V.
Because most power MOSFETs generally have a
maximum gate-source breakdown of 20V or less, the
use of a Zener clamp is recommended in applications
with VCC ≥ 8V. A Zener diode with 10V to 12V rating is
DS20006094A-page 24
recommended as shown in Figure 5-6. At the present
time, most power MOSFETs with a 20V gate-source
voltage rating have a 30V drain-source break-down
rating or higher. As a general tip, choose
surface-mount devices with a drain-source rating of
30V or more as a starting point.
Finally, the external gate drive of the MIC2085 requires
a low-voltage logic level MOSFET when operating at
voltage slower than 3V. There are 2.5V logic-level
MOSFETs available. Please see Table 5-1 and
Table 5-2 for suggested manufacturers.
5.10
MOSFET Steady-State Thermal
Issues
The selection of a MOSFET to meet the maximum
continuous current is a fairly straightforward exercise.
First, arm yourself with the following data:
• The value of ILOAD(CONT, MAX.) for the output in
question (see Sense Resistor Selection).
• The manufacturer’s data sheet for the candidate
MOSFET.
• The maximum ambient temperature in which the
device will be required to operate.
• Any knowledge you can get about the heat
sinking available to the device (e.g., can heat be
dissipated into the ground plane or power plane, if
using a surface-mount part? Is any airflow
available?).
The data sheet will almost always give a value of on
resistance given for the MOSFET at a gate-source
voltage of 4.5V, and another value at a gate-source
voltage of 10V. As a first approximation, add the two
values together and divide by two to get the
on-resistance of the part with 8V of enhancement. Call
this value RON. Because a heavily enhanced MOSFET
acts as an ohmic (resistive) device, almost all that’s
required to determine steady-state power dissipation is
to calculate I2R.The one addendum to this is that
MOSFETs have a slight increase in RON with
increasing die temperature. A good approximation for
this value is 0.5% increase in RON per °C rise in
junction temperature above the point at which RON was
initially specified by the manufacturer. For instance, if
the selected MOSFET has a calculated RON of 10 mΩ
at a TJ = 25°C, and the actual junction temperature
ends up at 110°C, a good first cut at the operating value
for RON would be:
EQUATION 5-12:
R ON 10m 1 + 110 – 25 0.005 14.3m
2019 Microchip Technology Inc.
MIC2085
• Airflow works. Even a few LFM (linear feet per
minute) of air will cool a MOSFET down
substantially. If you can, position the MOSFET(s)
near the inlet of a power supply’s fan, or the outlet
of a processor’s cooling fan.
• The best test of a surface-mount MOSFET for an
application (assuming the above tips show it to be
a likely fit) is an empirical one. Check the
MOSFET's temperature in the actual layout of the
expected final circuit, at full operating current. The
use of a thermocouple on the drain leads, or
infrared pyrometer on the package, will then give
a reasonable idea of the device’s junction
temperature.
The final step is to make sure that the heat sinking
available to the MOSFET is capable of dissipating at
least as much power (rated in °C/W) as that with which
the MOSFET’s performance was specified by the
manufacturer. Here are a few practical tips:
• The heat from a surface-mount device, such as
an SO-8 MOSFET, flows almost entirely out of the
drain leads. If the drain leads can be soldered
down to one square inch or more, the copper will
act as the heat sink for the part. This copper must
be on the same layer of the board as the
MOSFET drain.
Q1
IRF7822
(SO-8)
RSENSE
0.007Ω
1 2% 2
VIN
12V
3
*D1
1N5240B
10V
4
CLOAD
220μF
C1
1μF
R1
47kΩ
16
VCC
R3
10Ω
15
SENSE
14
GATE
ON
7
FB
MIC2085
R2
33kΩ
R5
13.3kΩ
1%
/FAULT
/POR
CPOR
3
6
5
Downstream
Signals
GND
8
C3
0.1μF
5.11
R4
100kΩ
1%
C2
0.01μF
4
FIGURE 5-6:
VOUT
12V@5A
Undervoltage (Output) = 11.0V
POR/START-UP Delay = 60ms
*Recommended for MOSFETs with gate-source
breakdown of 20V or less (IRF7822 VGS(MAX) = 12V)
for catastrophic output short circuit protection.
Additional pins omitted for clarity.
Zener-Clamped MOSFET Gate.
MOSFET Transient Thermal Issues
Having chosen a MOSFET that will withstand the
imposed voltage stresses, and the worse case
continuous I2R power dissipation which it will see, it
remains only to verify the MOSFET’s ability to handle
short-term overload power dissipation without
overheating. A MOSFET can handle a much higher
pulsed power without damage than its continuous
dissipation ratings would imply. The reason for this is
that, like everything else, thermal devices (silicon die,
lead frames, etc.) have thermal inertia.
In terms related directly to the specification and use of
power MOSFETs, this is known as “transient thermal
impedance,” or Zθ(JA). Almost all power MOSFET data
sheets give a Transient Thermal Impedance Curve. For
example, take the following case: VIN = 12V, tOCSLOW
has been set to 100 ms, ILOAD(CONT. MAX) is 2.5A, the
slow-trip threshold is 48 mV, nominal, and the fast-trip
2019 Microchip Technology Inc.
threshold is 95 mV. If the output is accidentally
connected to a 3Ω load, the output current from the
MOSFET will be regulated to 2.5A for 100 ms
(tOCSLOW) before the part trips. During that time, the
dissipation in the MOSFET is given by:
EQUATION 5-13:
P = EI
E MOSFET = 12V – 2.5 A 3 = 4.5V
P MOSFET = 4.5V 2.5 A = 11.25W for 100ms
At first glance, it would appear that a really hefty
MOSFET is required to withstand this sort of fault
condition. This is where the transient thermal
DS20006094A-page 25
MIC2085
impedance curves become very useful. Figure 5-7
shows the curve for the Vishay (Siliconix) Si4410DY, a
commonly used SO-8 power MOSFET.
EQUATION 5-14:
Taking the simplest case first, we’ll assume that once a
fault event such as the one in question occurs, it will be
a long time – 10 minutes or more – before the fault is
isolated and the channel is reset. In such a case, we
can approximate this as a “single pulse” event, that is
to say, there’s no significant duty cycle. Then, reading
up from the X-axis at the point where “Square Wave
Pulse Duration” is equal to 0.1 sec (100 ms), we see
that the Zθ(JA) of this MOSFET to a highly infrequent
event of this duration is only 8% of its continuous
Rθ(JA).
T J T A MAX +
This particular part is specified as having an Rθ(JA) of
50°C/W for intervals of 10 seconds or less. Thus:
Assume TA = 55°C maximum, 1 square inch of copper
at the drain leads, no airflow.
Recalling from our previous approximation hint, the
part has an RON of (0.0335/2) = 17 mΩ at 25°C.
Assume it has been carrying just about 2.5A for some
time.
When performing this calculation, be sure to use the
highest anticipated ambient temperature (TA(MAX)) in
which the MOSFET will be operating as the starting
temperature, and find the operating junction
temperature increase (∆TJ) from that point. Then, as
shown next, the final junction temperature is found by
adding TA(MAX) and ∆TJ. Because this is not a
closed-form equation, getting a close approximation
may take one or two iterations, but it’s not a hard
calculation to perform, and tends to converge quickly.
Then the starting (steady-state) TJ is:
T J T A MAX + T J
2
R ON + T A MAX – T A 0.005/C R ON I R JA
T J 55C + 17m + 55C – 25C 0.005 17m
2
2.5 A 50C/W
T J 55C + 0.122W 50C/W 61.1C
Iterate the calculation once to see if this value is within
a few percent of the expected final value. For this
iteration we will start with TJ equal to the already
calculated value of 61.1°C:
EQUATION 5-15:
TJ TA +
17m + 61.1C – 25C 0.005 17m 2.5 A
2
50C/W
T J 55C + 0.125W 50C/W 61.27C
So the original approximation of 61.1°C was very close
to the correct value. We will use TJ = 61°C.
Finally, add (11.25W)(50°C/W)(0.08) = 45°C to the
steady-state TJ to get TJ(TRANSIENT MAX.) = 106°C. This
is an acceptable maximum junction temperature for this
part.
Normalized Thermal Transient Impedance, Junction-to-Ambient
2
1
Normalized Effective Transient
Thermal Impedance
Duty Cycle = 0.5
0.2
Notes:
0.1
PDM
0.1
0.05
t1
t2
t1
1. Duty Cycle, D =
t2
2. Per Unit Base = R thJA = 50° C/W
0.02
3. TJM – TA = PDM ZthJA (t)
Single Pulse
4. Surface Mounted
0.01
10–4
10–3
10–2
10–1
1
10
30
Square Wave Pulse Duration (sec)
FIGURE 5-7:
DS20006094A-page 26
Transient Thermal Impedance.
2019 Microchip Technology Inc.
MIC2085
5.12
PCB Layout Considerations
feedback and overvoltage resistive networks are
selected for a12V application (from Figure 5-1). Many
hot swap applications will require load currents of
several amperes. Therefore, the power (VCC and
Return) trace widths (W) need to be wide enough to
allow the current to flow while the rise in temperature
for a given copper plate (e.g., 1 oz. or 2 oz.) is kept to
a maximum of 10°C ~ 25°C. Also, these traces should
be as short as possible in order to minimize the IR
drops between the input and the load.
Because of the low values of the sense resistors used
with the MIC2085 controller, special attention to the
layout must be used in order for the device’s circuit
breaker function to operate properly. Specifically, the
use of a 4-wire Kelvin connection to measure the
voltage across RSENSE is highly recommended. Kelvin
sensing is simply a means of making sure that any
voltage drops in the power traces connecting to the
resistors does not get picked up by the traces
themselves. Additionally, these Kelvin connections
should be isolated from all other signal traces to avoid
introducing noise onto these sensitive nodes.
Figure 5-8 illustrates a recommended, multi-layer
layout for the RSENSE, Power MOSFET, timer(s),
overvoltage and feedback network connections. The
Finally, plated-through vias are utilized to make circuit
connections to the power and ground planes. The trace
connections with indicated vias should follow the
example shown for the GND pin connection in
Figure 5-8.
Current Flow
to the Load
*POWER MOSFET
(SO-8)
*SENSE RESISTOR
(2512)
W
D
G
D
S
D
S
D
S
Current Flow
to the Load
W
**R4
ȍ
Via to GND Plane
**CGATE
Current Flow
from the Load
Via to GND Plane
OV 9
R2
Nȍ
1%
Via to POWER (VCC)
Plane
8 GND
COMPOUT 10
COMP+ 11
6 /FAULT
**CPOR
7 FB
COMP– 12
5 /POR
REF 13
ON
4
GATE 14
SENSE 15
2 CFILTER
**CFILTER
3 CPOR
VCC 16
1 CRWBR
MIC2085
R7
Nȍ
1%
R5
Nȍ
1%
R6
Nȍ
1%
W
DRAWING IS NOT TO SCALE
*See Table 5-1 and 5-2 for part numbers and vendors.
**Optional components.
Trace width (W) guidelines given in “PCB Layout Recommendations” section of the data sheet.
FIGURE 5-8:
Recommended PCB Layout for Sense Resistor, Power MOSFET, and
Feedback/Overvoltage Network.
2019 Microchip Technology Inc.
DS20006094A-page 27
MIC2085
5.13
MOSFET and Sense Resistor Vendors
Device types and manufacturer contact information for power MOSFETs and sense resistors is provided in Table 5-1
and Table 5-2. Some of the recommended MOSFETs include a metal heat sink on the bottom side of the package. The
recommended trace for the MOSFET Gate of Figure 5-8 must be redirected when using MOSFETs packaged in this
style. Contact the device manufacturer for package information.
TABLE 5-1:
MOSFET VENDORS
Vendor
Key MOSFET Type(s)
Vishay (Siliconix)
International Rectifier
Contact Information
Si4420DY (SO-8 package)
IOUT ≤ 10A
Si4442DY (SO-8 package)
IOUT = 10A – 15A, VCC ≤ 5V
Si3442DV (SO-8 package)
IOUT ≤ 3A, VCC ≤ 5V
Si7860DP (PowerPAK™ SO-8)
IOUT ≤ 12A
Si7892DP (PowerPAK™ SO-8)
IOUT ≤ 15A
Si7884DP (PowerPAK™ SO-8)
IOUT ≤ 15A
SUB60N06-18 (TO-263)
IOUT ≥ 20A, VCC ≥ 5V
www.siliconix.com
(203) 452-5664
SUB70N04-10 (TO-263)
IOUT ≥ 20A, VCC ≥ 5V
IRF7413 (SO-8 package)
IOUT ≤ 10A
IRF7457 (SO-8 package)
IOUT ≤ 10A
IRF7822 (SO-8 package)
IOUT = 10A – 15A, VCC ≤ 5V
IRLBA1304 (Super220™)
IOUT ≥ 20A, VCC ≥ 5V
FDS6680A (SO-8 package)
IOUT ≤ 10A
www.irf.com
(310) 322-3331
FDS6690A (SO-8 package)
IOUT ≥ 10A, VCC ≥ 5V
www.fairchildsemi.com
(207) 775-8100
Philips
PH3230 (SOT669-LFPAK)
IOUT ≥ 20A
www.philips.com
Hitachi
HAT2099H (LFPAK)
IOUT ≥ 20A
www.halsp.hitachi.com
(408) 433-1990
Fairchild Semiconductor
Note 1:
Applications (Note 1)
These devices are not limited to these conditions in many cases, but these conditions are provided as a
helpful reference for customer applications.
TABLE 5-2:
SENSE RESISTOR VENDORS
Vendor
Sense Resistors
Contact Information
Vishay (Dale)
“WSL” Series
(203) 452-5664
IRC
“OARS” Series
”LR” Series
(second source to “WSL”)
(828) 264-8861
DS20006094A-page 28
2019 Microchip Technology Inc.
MIC2085
6.0
PACKAGING INFORMATION
6.1
Package Marking Information
16-Lead QSOP*
XXXX
-XXXX
WNNN
Legend: XX...X
Y
YY
WW
NNN
e3
*
Example
2085
-JYQS
8626
Product code or customer-specific information
Year code (last digit of calendar year)
Year code (last 2 digits of calendar year)
Week code (week of January 1 is week ‘01’)
Alphanumeric traceability code
Pb-free JEDEC® designator for Matte Tin (Sn)
This package is Pb-free. The Pb-free JEDEC designator ( e3 )
can be found on the outer packaging for this package.
●, ▲, ▼ Pin one index is identified by a dot, delta up, or delta down (triangle
mark).
Note:
In the event the full Microchip part number cannot be marked on one line, it will
be carried over to the next line, thus limiting the number of available
characters for customer-specific information. Package may or may not include
the corporate logo.
Underbar (_) and/or Overbar (‾) symbol may not be to scale.
2019 Microchip Technology Inc.
DS20006094A-page 29
MIC2085
16-Lead QSOP Package Outline & Recommended Land Pattern
Note:
For the most current package drawings, please see the Microchip Packaging Specification located at
http://www.microchip.com/packaging.
DS20006094A-page 30
2019 Microchip Technology Inc.
MIC2085
APPENDIX A:
REVISION HISTORY
Revision A (June 2019)
• Converted Micrel document MIC2085 to Microchip data sheet template DS20006094A.
• Minor grammatical text changes throughout.
• All reference to and information about the
MIC2086 has been removed.
2019 Microchip Technology Inc.
DS20006094A-page 31
MIC2085
NOTES:
DS20006094A-page 32
2019 Microchip Technology Inc.
MIC2085
PRODUCT IDENTIFICATION SYSTEM
To order or obtain information, e.g., on pricing or delivery, contact your local Microchip representative or sales office.
Examples:
Device
-X
X
XX
-XX
Part No.
Feature
Junction
Temp. Range
Package
Media Type
Device:
MIC2085:
Single-Channel Low Voltage Hot Swap
Controller
Feature:
Fast Circuit Breaker Threshold
J
=
95mV
K
=
150mV*
L
=
200mV*
M
=
Off
Junction
Temperature
Range:
Y
Package:
QS =
Media Type:
= 98/Tube
TR =
2,500/Reel
=
a) MIC2085-JYQS:
MIC2085, 95mV Fast Circuit
Breaker Threshold, –40°C to
+85°C Temperature Range,
16-Lead QSOP, 98/Tube
b) MIC2085-KYQS:
MIC2085, 150mV* Fast Circuit
Breaker Threshold, –40°C to
+85°C Temperature Range,
16-Lead QSOP, 98/Tube
c) MIC2085-LYQS-TR:
MIC2085, 200mV* Fast Circuit
Breaker Threshold, –40°C to
+85°C Temperature Range,
16-Lead QSOP, 2,500/Reel
d) MIC2085-MYQS-TR:
MIC2085, Fast Circuit Breaker
Threshold OFF, –40°C to
+85°C Temperature Range,
16-Lead QSOP, 2,500/Reel
–40°C to +85°C, RoHS-Compliant
16-Lead QSOP
Note 1:
Tape and Reel identifier only appears in the
catalog part number description. This identifier is
used for ordering purposes and is not printed on
the device package. Check with your Microchip
Sales Office for package availability with the
Tape and Reel option.
* Contact factory for availabilty.
2019 Microchip Technology Inc.
DS20006094A-page 33
MIC2085
NOTES:
DS20006094A-page 34
2019 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is provided only for your convenience
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
MICROCHIP MAKES NO REPRESENTATIONS OR
WARRANTIES OF ANY KIND WHETHER EXPRESS OR
IMPLIED, WRITTEN OR ORAL, STATUTORY OR
OTHERWISE, RELATED TO THE INFORMATION,
INCLUDING BUT NOT LIMITED TO ITS CONDITION,
QUALITY, PERFORMANCE, MERCHANTABILITY OR
FITNESS FOR PURPOSE. Microchip disclaims all liability
arising from this information and its use. Use of Microchip
devices in life support and/or safety applications is entirely at
the buyer’s risk, and the buyer agrees to defend, indemnify and
hold harmless Microchip from any and all damages, claims,
suits, or expenses resulting from such use. No licenses are
conveyed, implicitly or otherwise, under any Microchip
intellectual property rights unless otherwise stated.
Trademarks
The Microchip name and logo, the Microchip logo, Adaptec,
AnyRate, AVR, AVR logo, AVR Freaks, BesTime, BitCloud, chipKIT,
chipKIT logo, CryptoMemory, CryptoRF, dsPIC, FlashFlex,
flexPWR, HELDO, IGLOO, JukeBlox, KeeLoq, Kleer, LANCheck,
LinkMD, maXStylus, maXTouch, MediaLB, megaAVR, Microsemi,
Microsemi logo, MOST, MOST logo, MPLAB, OptoLyzer,
PackeTime, PIC, picoPower, PICSTART, PIC32 logo, PolarFire,
Prochip Designer, QTouch, SAM-BA, SenGenuity, SpyNIC, SST,
SST Logo, SuperFlash, Symmetricom, SyncServer, Tachyon,
TempTrackr, TimeSource, tinyAVR, UNI/O, Vectron, and XMEGA
are registered trademarks of Microchip Technology Incorporated in
the U.S.A. and other countries.
APT, ClockWorks, The Embedded Control Solutions Company,
EtherSynch, FlashTec, Hyper Speed Control, HyperLight Load,
IntelliMOS, Libero, motorBench, mTouch, Powermite 3, Precision
Edge, ProASIC, ProASIC Plus, ProASIC Plus logo, Quiet-Wire,
SmartFusion, SyncWorld, Temux, TimeCesium, TimeHub,
TimePictra, TimeProvider, Vite, WinPath, and ZL are registered
trademarks of Microchip Technology Incorporated in the U.S.A.
Adjacent Key Suppression, AKS, Analog-for-the-Digital Age, Any
Capacitor, AnyIn, AnyOut, BlueSky, BodyCom, CodeGuard,
CryptoAuthentication, CryptoAutomotive, CryptoCompanion,
CryptoController, dsPICDEM, dsPICDEM.net, Dynamic Average
Matching, DAM, ECAN, EtherGREEN, In-Circuit Serial
Programming, ICSP, INICnet, Inter-Chip Connectivity, JitterBlocker,
KleerNet, KleerNet logo, memBrain, Mindi, MiWi, MPASM, MPF,
MPLAB Certified logo, MPLIB, MPLINK, MultiTRAK, NetDetach,
Omniscient Code Generation, PICDEM, PICDEM.net, PICkit,
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Blocker, SAM-ICE, Serial Quad I/O, SMART-I.S., SQI,
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USBCheck, VariSense, ViewSpan, WiperLock, Wireless DNA, and
ZENA are trademarks of Microchip Technology Incorporated in the
U.S.A. and other countries.
SQTP is a service mark of Microchip Technology Incorporated in
the U.S.A.
The Adaptec logo, Frequency on Demand, Silicon Storage
Technology, and Symmcom are registered trademarks of Microchip
Technology Inc. in other countries.
GestIC is a registered trademark of Microchip Technology Germany
II GmbH & Co. KG, a subsidiary of Microchip Technology Inc., in
other countries.
All other trademarks mentioned herein are property of their
respective companies.
© 2019, Microchip Technology Incorporated, All Rights Reserved.
For information regarding Microchip’s Quality Management Systems,
please visit www.microchip.com/quality.
2019 Microchip Technology Inc.
ISBN: 978-1-5224-4579-1
DS20006094A-page 35
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DS20006094A-page 36
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05/14/19
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