MIC45205
26V/6A DC/DC Power Module
Features
General Description
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MIC45205 is a synchronous step-down regulator
module, featuring a unique adaptive ON-time control
architecture. The module incorporates a DC/DC
controller, power MOSFETs, bootstrap diode, bootstrap
capacitor, and an inductor in a single package;
simplifying the design and layout process for the end
user.
•
•
•
•
•
•
•
•
•
•
No Compensation Required
Up to 6A Output Current
>93% Peak Efficiency
Output Voltage: 0.8V to 0.85 x VIN with ±1%
Accuracy
Adjustable Switching Frequency from 200 kHz to
600 kHz
Enable Input and Open-Drain Power Good Output
HyperLight Load (MIC45205-1) Improves Light
Load Efficiency
Hyper Speed Control (MIC45205-2) Architecture
Enables Fast Transient Response
Supports Safe Startup into Pre-Biased Output
–40°C to +125°C Junction Temperature Range
Thermal Shutdown Protection
Short-Circuit Protection with Hiccup Mode
Adjustable Current-Limit
Available in 52-pin 8 mm × 8 mm × 3 mm B1QFN
Package
This highly integrated solution expedites system
design and improves product time-to-market. The
internal MOSFETs and inductor are optimized to
achieve high efficiency at a low output voltage. The fully
optimized design can deliver up to 6A current under a
wide input voltage range of 4.5V to 26V, without
requiring additional cooling.
The MIC45205-1 uses Microchip’s HyperLight Load®
(HLL) and the MIC45205-2 uses Microchip’s Hyper
Speed Control® architecture that enables ultra-fast
load transient response, allowing for a reduction of
output capacitance.
The MIC45205 offers 1% output accuracy that can be
adjusted from 0.8V to 0.85 x VIN with two external
resistors. Additional features include thermal shutdown
protection, input undervoltage lockout, adjustable
current-limit, and short-circuit protection. The
MIC45205 allows for safe start-up into a pre-biased
output.
Applications
• High Power Density Point-of-Load Conversion
• Servers, Routers, Networking, and Base Stations
• FPGAs, DSP, and Low-Voltage ASIC Power
Supplies
• Industrial and Medical Equipment
Typical Application Diagram
VIN
12V
PVDD
ANODE
5VDD
BST
PG
RIA
VOUT
PVIN
VIN
CIN
MIC45205
FREQ
ON
FB
RIB
CFF
EN
RFB1
COUT
RFB2
SW
RLIM
OFF
GND
2017 - 2022 Microchip Technology Inc.
VOUT
UP to 6A
ILIM
PGND
DS20005798B-page 1
MIC45205
Functional Block Diagram
BST
VIN
5VDD
VDD
VIN
ANODE
BST
PVIN
PVDD
EN
FREQ
RIB
PVDD
PWM
CONTROLLER
DH
EN
FREQ
SW
PG
FB
GND
RINJ
RIA
CINJ
SW
VOUT
DL
PG
FB
AGND
ILIM
PGND
PGND
ILIM
DS20005798B-page 2
2017 - 2022 Microchip Technology Inc.
MIC45205
1.0
ELECTRICAL CHARACTERISTICS
Absolute Maximum Ratings †
VPVIN, VVIN to PGND ................................................................................................................................. –0.3V to +30V
VPVDD, V5VDD, VANODE to PGND................................................................................................................. –0.3V to +6V
VSW, VFREQ, VILIM, VEN to PGND ....................................................................................................–0.3V to (VIN + 0.3V)
VBST to VSW ................................................................................................................................................. –0.3V to +6V
VBST to PGND............................................................................................................................................ –0.3V to +36V
VPG to PGND ............................................................................................................................... –0.3V to (5VDD + 0.3V)
VFB, VRIB to PGND ...................................................................................................................... –0.3V to (5VDD + 0.3V)
PGND to GND........................................................................................................................................... –0.3V to +0.3V
Operating Ratings ‡
Supply Voltage (VPVIN, VVIN) ..................................................................................................................... +4.5V to +26V
Output Current ..............................................................................................................................................................6A
Enable Input (VEN) ..............................................................................................................................................0V to VIN
Power Good (VPG) .......................................................................................................................................... 0V to 5VDD
† Notice: Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device.
This is a stress rating only and functional operation of the device at those or any other conditions above those indicated
in the operational sections of this specification is not intended. Exposure to maximum rating conditions for extended
periods may affect device reliability.
‡ Notice: The device is not guaranteed to function outside its operating ratings.
ELECTRICAL CHARACTERISTICS
Electrical Characteristics: VIN = VEN = 12V, VOUT = 3.3V, VBST – VSW = 5V, TJ = +25ºC. Bold values indicate –40ºC
< TJ < +125ºC, unless otherwise noted. Note 1
Parameter
Symbol
Min.
Typ.
Max.
Units
Conditions
Power Supply Input
VIN, PVIN
4.5
—
26
V
Quiescent Supply Current
(MIC45205-1)
IQ
—
0.35
0.75
mA
VFB = 1.5V
Quiescent Supply Current
(MIC45205-2)
IQ
—
2.1
3
mA
VFB = 1.5V
Operating Current
IIN
—
31
—
mA
ISHDN
—
0.1
10
VPVIN = VIN = 12V, VOUT = 1.8V,
IOUT = 0A
fSW = 600 kHz (MIC45205-2)
µA
SW = unconnected, VEN = 0V
5VDD Output Voltage
5VDD
4.8
5.1
5.4
V
UVLO
3.8
4.2
4.6
VIN = 7V to 26V, I5VDD = 10 mA
5VDD UVLO Threshold
V
V5VDD rising
5VDD UVLO Hysteresis
UVLO_HYS
—
400
—
mV
Δ5VDD(LR)
0.6
2
3.6
%
0.792
0.8
0.808
Input Voltage Range
Shutdown Supply Current
5VDD Output
LDO Load Regulation
Reference
Feedback Reference Voltage
VREF
V
—
V5VDD falling
I5VDD = 0 mA to 40 mA
TJ = +25°C
0.784
0.8
0.816
IFB_BIAS
—
5
500
nA
VFB = 0.8V
EN Logic Level High
ENHIGH
1.8
—
—
V
—
EN Logic Level Low
ENLOW
—
—
0.6
V
—
FB Bias Current
Enable Control
2017 - 2022 Microchip Technology Inc.
–40°C ≤ TJ ≤ +125°C
DS20005798B-page 3
MIC45205
ELECTRICAL CHARACTERISTICS (CONTINUED)
Electrical Characteristics: VIN = VEN = 12V, VOUT = 3.3V, VBST – VSW = 5V, TJ = +25ºC. Bold values indicate –40ºC
< TJ < +125ºC, unless otherwise noted. Note 1
Parameter
Symbol
Min.
Typ.
EN Hysteresis
ENHYS
—
EN Bias Current
IENBIAS
—
Oscillator
Switching Frequency
fSW
Maximum Duty Cycle
DMAX
Minimum Duty Cycle
Minimum Off-Time
Soft-Start
Soft-Start Time
Short-Circuit Protection
Current-Limit Threshold
Max.
Units
200
—
mV
—
5
10
µA
VEN = 12V
400
600
750
—
350
—
—
85
—
kHz
Conditions
VFREQ = VIN, IOUT = 2A
VFREQ = 50% VIN, IOUT = 2A
%
—
DMIN
—
0
—
%
VFB = 1V
tOFF(MIN)
140
200
260
ns
—
tSS
—
5
—
ms
VFB from 0V to 0.8V
VCL_
–30
–14
0
mV
VFB = 0.79V
OFFSET
Short-Circuit Threshold
VSC
–23
–7
9
mV
VFB = 0V
Current-Limit Source Current
ICL
55
70
85
µA
VFB = 0.79V
Short-Circuit Source Current
ISC
25
35
45
µA
VFB = 0V
ISW_
—
—
10
µA
—
—
—
10
µA
—
Leakage
SW, BST Leakage Current
FREQ Leakage Current
Power Good (PG)
LEAKAGE
IFREQ_
LEAK
PG Threshold Voltage
VPG_TH
85
90
95
% VREF
Sweep VFB from Low-to-High
PG Hysteresis
VPG_HYS
—
6
—
% VREF
Sweep VFB from High-to-Low
PG Delay Time
tPG_DLY
—
100
—
µs
VPG_LOW
—
70
200
Sweep VFB from Low-to-High
PG Low Voltage
mV
VFB < 90% × VNOM, IPG = 1 mA
Overtemperature Shutdown
TSHD
—
160
—
°C
TJ rising
Overtemperature Shutdown
Hysteresis
TSHD_
—
15
—
°C
—
Thermal Protection
Note 1:
HYS
Specification for packaged product only.
DS20005798B-page 4
2017 - 2022 Microchip Technology Inc.
MIC45205
TEMPERATURE SPECIFICATIONS (Note 1)
Parameters
Sym.
Min.
Typ.
Max.
Units
Conditions
TJ
–40
—
+125
°C
—
TJ(ABSMAX)
—
—
+150
°C
—
Temperature Ranges
Operating Junction Temperature
Range
Absolute Maximum Junction
Temperature
TS
–65
—
+150
°C
—
TLEAD
—
—
+260
°C
Soldering, 10s
Thermal Resistance B1QFN-52
JA
—
21.7
—
°C/W
Note 2
Thermal Resistance B1QFN-52
JC
—
5.0
—
°C/W
Note 2
Storage Temperature Range
Lead Temperature
Package Thermal Resistances
Note 1:
2:
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.
θJA and θJC were measured using the MIC45205 evaluation board.
2017 - 2022 Microchip Technology Inc.
DS20005798B-page 5
MIC45205
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.
FIGURE 2-1:
VIN Operating Supply
Current vs. Temperature (MIC45205-1).
FIGURE 2-4:
Temperature.
EN Bias Current vs.
FIGURE 2-2:
Temperature.
5VDD Supply Voltage vs.
FIGURE 2-5:
Temperature.
Feedback Voltage vs.
FIGURE 2-3:
Temperature.
Enable Threshold vs.
FIGURE 2-6:
Temperature.
Output Voltage vs.
DS20005798B-page 6
2017 - 2022 Microchip Technology Inc.
MIC45205
FIGURE 2-7:
Temperature.
Switching Frequency vs.
FIGURE 2-10:
Efficiency (VIN = 12V) vs.
Output Current (MIC45205-1).
FIGURE 2-8:
vs. Temperature.
Output Peak Current Limit
FIGURE 2-11:
Efficiency (VIN = 24V) vs.
Output Current (MIC45205-1).
FIGURE 2-9:
Efficiency (VIN = 5V) vs.
Output Current (MIC45205-1).
2017 - 2022 Microchip Technology Inc.
FIGURE 2-12:
Efficiency (VIN = 5V) vs.
Output Current (MIC45205-2).
DS20005798B-page 7
MIC45205
FIGURE 2-13:
Efficiency (VIN = 12V) vs.
Output Current (MIC45205-2).
FIGURE 2-16:
Voltage.
FIGURE 2-14:
Efficiency (VIN = 24V) vs.
Output Current (MIC45205-2).
FIGURE 2-17:
IC Power Dissipation (VIN =
5V) vs. Output Current.
FIGURE 2-15:
FIGURE 2-18:
IC Power Dissipation (VIN =
12V) vs. Output Current.
DS20005798B-page 8
Line Regulation.
Output Voltage vs. Input
2017 - 2022 Microchip Technology Inc.
MIC45205
VEN
(2V/div)
VIN = 12V
VOUT = 1.8V
IOUT = 6A
VOUT
(1V/div)
IIN
(1A/div)
Time (2ms/div)
FIGURE 2-19:
IC Power Dissipation (VIN =
24V) vs. Output Current.
VIN
(10V/div)
VOUT
(1V/div)
FIGURE 2-22:
Rise Time.
VEN
(2V/div)
VIN = 12V
VOUT = 1.8V
IOUT = 6A
VIN = 12V
VOUT = 1.8V
IOUT = 6A
VOUT
(1V/div)
PGOOD
(5V/div)
IIN
(1A/div)
IIN
(2A/div)
Time (2ms/div)
Time (2ms/div)
FIGURE 2-20:
Enable Turn-On Delay and
VIN Soft Turn-On.
VIN
(10V/div)
VIN = 12V
VOUT = 1.8V
IOUT = 6A
VOUT
(1V/div)
FIGURE 2-23:
Fall Time.
Enable Turn-Off Delay and
VIN
(10V/div)
VOUT
(1V/div)
PGOOD
(5V/div)
PGOOD
(5V/div)
VIN = 12V
VOUT = 1.8V
IOUT = 1A
VPRE-BIAS = 0.5V
IIN
(2A/div)
Time (2ms/div)
FIGURE 2-21:
VIN Soft Turn-Off.
2017 - 2022 Microchip Technology Inc.
Time (2ms/div)
FIGURE 2-24:
VIN Start-Up with
Pre-Biased Output.
DS20005798B-page 9
MIC45205
VEN
(2V/div)
VOUT
(1V/div)
VOUT
(1V/div)
VIN = 12V
VOUT = 1.8V
IOUT = 6A
VIN = 12V
VOUT = 1.8V
IOUT
(5A/div)
IIN
(1A/div)
Time (8ms/div)
FIGURE 2-25:
VIN
(10V/div)
Enable Turn-On/Off.
VIN = 12V, VOUT = 1.8V
IOUT = SHORT CIRCUIT
Time (2ms/div)
FIGURE 2-28:
VOUT
(1V/div)
VOUT
(20mV/div)
VIN = 12V
VOUT = 1.8V
IOUT
(5A/div)
IIN
(500mA/div)
Time (2ms/div)
FIGURE 2-26:
VEN
(2V/div)
Power-Up Into Short-Circuit.
VIN = 12V
VOUT = 1.8V
IOUT = SHORT CIRCUIT
VOUT
(20mV/div)
IIN
(100mA/div)
Time (8ms/div)
FIGURE 2-29:
Short-Circuit.
DS20005798B-page 10
Output Recovery from
VOUT
(1V/div)
VIN = 12V
VOUT = 1.8V
IPK_CL = 8.4A
IOUT
(5A/div)
Time (400μs/div)
FIGURE 2-27:
Short-Circuit.
Enabled Into Short-Circuit.
Time (4ms/div)
FIGURE 2-30:
Threshold.
Peak Current-Limit
2017 - 2022 Microchip Technology Inc.
MIC45205
VIN = 12V
VOUT = 1.8V
IOUT = 6A
VOUT
(1V/div)
VOUT
(1V/div)
VSW
(5V/div)
IIN
(1A/div)
PG
(5V/div)
Time (8ms/div)
Time (2ms/div)
FIGURE 2-31:
Short-Circuit.
Output Recovery from
VIN = 12V
VOUT = 1.8V
IOUT = 6A
FIGURE 2-34:
Inrush with COUT = 3000 µF.
VOUT
(100mV/div)
VOUT
(AC-COUPLED)
(20mV/div)
VSW
(5V/div)
IOUT
(2A/div)
IOUT
(5A/div)
Time (1μs/div)
FIGURE 2-32:
= 6A).
VIN = 12V
VOUT = 1.8V
IOUT = 6A
VEN
(2V/div)
VIN = 12V
VOUT = 1.8V
IOUT = 3A TO 6A
Time (100μs/div)
Switching Waveforms (IOUT
FIGURE 2-35:
Load Transient Response,
MIC45205-1 (IOUT = 3A to 6A).
VOUT
(100mV/div)
VOUT
(AC-COUPLED)
(20mV/div)
VSW
(5V/div)
VIN = 12V
VOUT = 1.8V
IOUT = 0A
IOUT
(2A/div)
Time (20ms/div)
FIGURE 2-33:
Switching Waveforms,
MIC45205-1 (IOUT = 0A).
2017 - 2022 Microchip Technology Inc.
IOUT
(2A/div)
VIN = 12V
VOUT = 1.8V
IOUT = 0.5A TO 3.5A
Time (100μs/div)
FIGURE 2-36:
Load Transient Response,
MIC45205-1 (IOUT = 0.5A to 3.5A).
DS20005798B-page 11
MIC45205
42 BST
43 BST
44 BST
45 PGND
46 FB
47 PG
48 EN
49 VIN
50 FREQ
51 ILIM
PIN DESCRIPTIONS
52 PGND
3.0
GND 1
41 ANODE
5VDD 2
40 ANODE
5VDD 3
39 RIB
PVDD 4
38 RIA
PGND
5
37 RIA
PGND 6
36 RIA
PGND 7
35 KEEPOUT
PGND 8
34 SW
PVDD
33 SW
KEEPOUT 9
SW
32 SW
SW 10
31 SW
SW 11
30 KEEPOUT
SW 12
KEEPOUT 13
FIGURE 3-1:
PVIN ePAD
29 VOUT
VOUT ePAD
VOUT 26
VOUT 25
VOUT 24
VOUT 23
VOUT 22
VOUT 21
KEEPOUT 20
PVIN 19
PVIN 18
PVIN 15
PVIN 17
28 VOUT
27 VOUT
PVIN 16
PVIN 14
MIC45205 Pin Configuration.
The descriptions of the pins are listed in Table 3-1.
TABLE 3-1:
PIN FUNCTION TABLE
Pin Number
Pin Name
1
GND
Analog Ground. Connect bottom feedback resistor to GND. GND and PGND are
internally connected.
2, 3
5VDD
Internal +5V Linear Regulator Output. Powered by VIN, 5VDD is the internal supply
bus for the device. In the applications with VIN < +5.5V, 5VDD should be tied to VIN to
bypass the linear regulator.
4, 5
PVDD
PVDD. Supply input for the internal low-side power MOSFET driver.
PGND
Power Ground. PGND is the return path for the step-down power module power
stage. The PGND pin connects to the sources of internal low-side power MOSFET,
the negative terminals of input capacitors, and the negative terminals of output
capacitors.
6, 7, 8, 45, 52
Description
10, 11, 12, 31,
32, 33, 34
SW
The SW pin connects directly to the switch node. Due to the high-speed switching on
this pin, the SW pin should be routed away from sensitive nodes. The SW pin also
senses the current by monitoring the voltage across the low-side MOSFET during
OFF time.
14, 15, 16, 17,
18, 19
PVIN
Power Input Voltage. Connection to the drain of the internal high-side power
MOSFET. Connect an input capacitor from PVIN to PGND.
21, 22, 23, 24,
25, 26, 27, 28,
29
VOUT
Output Voltage. Connected to the internal inductor, the output capacitor should be
connected from this pin to PGND as close to the module as possible.
36, 37, 38
RIA
DS20005798B-page 12
Ripple Injection Pin A. Leave floating, no connection.
2017 - 2022 Microchip Technology Inc.
MIC45205
TABLE 3-1:
PIN FUNCTION TABLE (CONTINUED)
Pin Number
Pin Name
Description
39
RIB
40, 41
ANODE
42, 43, 44
BST
46
FB
Feedback. Input to the transconductance amplifier of the control loop. The FB pin is
referenced to 0.8V. A resistor divider connecting the feedback to the output is used to
set the desired output voltage. Connect the bottom resistor from FB to GND.
47
PG
Power Good. Open-drain output. If used, connect to an external pull-up resistor of at
least 10 kΩ between PG and the external bias voltage.
48
EN
Enable. A logic signal to enable or disable the step-down regulator module operation.
The EN pin is TTL/CMOS compatible. Logic-high = enable, logic-low = disable or
shutdown. EN pin has an internal 1 MΩ (typical) pull-down resistor to GND. Do not
leave floating.
49
VIN
Internal 5V Linear Regulator Input. A 1 μF ceramic capacitor from VIN to GND is
required for decoupling.
50
FREQ
Switching Frequency Adjust. Use a resistor divider from VIN to GND to program the
switching frequency. Connecting FREQ to VIN sets frequency at 600 kHz.
51
ILIM
Current Limit. Connect a resistor between ILIM and SW to program the current limit.
Ripple Injection Pin B. Connect this pin to FB.
Anode Bootstrap Diode. Anode connection of internal bootstrap diode, this pin should
be connected to the PVDD pin.
Connection to the internal bootstrap circuitry and high-side power MOSFET drive
circuitry. Connect all three BST pins together.
9, 13, 20, 30, 35 KEEPOUT Depopulated pin positions.
—
PVIN ePAD
PVIN Exposed Pad. Internally connected to PVIN pins. Please see PCB Layout
Guidelines section.
—
VOUT
ePAD
VOUT Exposed Pad. Internally connected to VOUT pins. Please see PCB Layout
Guidelines section.
2017 - 2022 Microchip Technology Inc.
DS20005798B-page 13
MIC45205
4.0
FUNCTIONAL DESCRIPTION
The MIC45205 is an adaptive on-time synchronous
buck regulator module built for high-input voltage to
low-output voltage conversion applications. The
MIC45205 is designed to operate over a wide input
voltage range, from 4.5V to 26V, and the output is
adjustable with an external resistor divider. An adaptive
on-time control scheme is employed to obtain a
constant switching frequency in steady state and to
simplify the control compensation. Hiccup mode
overcurrent protection is implemented by sensing
low-side MOSFET’s RDS(ON). The device features
internal soft-start, enable, UVLO, and thermal
shutdown. The module has integrated switching FETs,
inductor, bootstrap diode, resistor, and capacitor.
4.1
As shown in Figure 4-1 (in association with
Equation 4-1), the output voltage is sensed by the
MIC45205 feedback pin (FB) via the voltage divider
RFB1 and RFB2 and compared to a 0.8V reference
voltage (VREF) at the error comparator through a
low-gain transconductance (gm) amplifier. If the
feedback voltage decreases, and the amplifier output
falls below 0.8V, then the error comparator will trigger
the control logic and generate an ON-time period. The
ON-time period length is predetermined by the “Fixed
tON Estimator” circuitry:
VOUT
COMPENSATION
RFB1
FB
COMP
RFB2
VREF
0.8V
FIGURE 4-1:
FB Pin.
Output Voltage Sense via
EQUATION 4-1:
V OUT
t ON ESTIMATED = ---------------------V IN f SW
Where:
VOUT
Output Voltage
VIN
Power Stage Input Voltage
fSW
Switching Frequency
DS20005798B-page 14
The maximum duty cycle is obtained from the 200 ns
tOFF(MIN):
EQUATION 4-2:
Theory of Operation
gM EA
At the end of the ON-time period, the internal high-side
driver turns off the high-side MOSFET and the low-side
driver turns on the low-side MOSFET. The OFF-time
period length depends upon the feedback voltage in
most cases. When the feedback voltage decreases
and the output of the gm amplifier falls below 0.8V, the
ON-time period is triggered and the OFF-time period
ends. If the OFF-time period determined by the
feedback voltage is less than the minimum OFF-time
tOFF(MIN), which is about 200 ns, the MIC45205 control
logic will apply the tOFF(MIN) instead. tOFF(MIN) is
required to maintain enough energy in the boost
capacitor (CBST) to drive the high-side MOSFET.
t S – t OFF MIN
D MAX = ---------------------------------- = 1 – 200ns
--------------tS
tS
Where:
tS
1/fSW
It is not recommended to use MIC45205 with an
OFF-time close to tOFF(MIN) during steady-state
operation.
The adaptive ON-time control scheme results in a
constant switching frequency in the MIC45205 during
steady state operation. The actual ON-time and
resulting switching frequency will vary with the different
rising and falling times of the MOSFETs. Also, the
minimum tON results in a lower switching frequency in
high VIN to VOUT applications. During load transients,
the switching frequency is changed due to the varying
OFF-time.
To illustrate the control loop operation, we will analyze
both the steady-state and load transient scenarios. For
easy analysis, the gain of the gm amplifier is assumed
to be 1. With this assumption, the inverting input of the
error comparator is the same as the feedback voltage.
Figure 4-2 shows the MIC45205 control loop timing
during steady-state operation. During steady-state, the
gm amplifier senses the feedback voltage ripple, which
is proportional to the output voltage ripple plus injected
voltage ripple, to trigger the ON-time period. The
ON-time is predetermined by the tON estimator. The
termination of the OFF-time is controlled by the
feedback voltage. At the valley of the feedback voltage
ripple, which occurs when VFB falls below VREF, the
OFF period ends and the next ON-time period is
triggered through the control logic circuitry.
2017 - 2022 Microchip Technology Inc.
MIC45205
Unlike true current-mode control, the MIC45205 uses
the output voltage ripple to trigger an ON-time period.
The output voltage ripple is proportional to the inductor
current ripple if the ESR of the output capacitor is large
enough.
VDH
FIGURE 4-2:
Timing.
MIC45205 Control Loop
Figure 4-3 shows the operation of the MIC45205 during
a load transient. The output voltage drops due to the
sudden load increase, which causes the VFB to be less
than VREF. This will cause the error comparator to
trigger an ON-time period. At the end of the ON-time
period, a minimum OFF-time tOFF(MIN) is generated to
charge the bootstrap capacitor (CBST) because the
feedback voltage is still below VREF. Then, the next
ON-time period is triggered due to the low feedback
voltage. Therefore, the switching frequency changes
during the load transient, but returns to the nominal
fixed frequency once the output has stabilized at the
new load current level. With the varying duty cycle and
switching frequency, the output recovery time is fast
and the output voltage deviation is small. Note that the
instantaneous switching frequency during load
transient remains bounded and cannot increase
arbitrarily. The minimum is limited by tON + tOFF(MIN).
Because the variation in VOUT is relatively limited
during load transient, tON stays virtually close to its
steady-state value.
4.2
Discontinuous Mode (MIC45205-1
Only)
In continuous mode, the inductor current is always
greater than zero. However, at light loads, the
MIC45205-1 is able to force the inductor current to
operate in discontinuous mode. Discontinuous mode is
where the inductor current falls to zero, as indicated by
trace (IL) shown in Figure 4-4. During this period, the
efficiency is optimized by shutting down all the
non-essential circuits and minimizing the supply
current as the switching frequency is reduced. The
MIC45205-1 wakes up and turns on the high-side
MOSFET when the feedback voltage VFB drops below
0.8V.
The MIC45205-1 has a zero crossing comparator (ZC)
that monitors the inductor current by sensing the
voltage drop across the low-side MOSFET during its
ON-time. If the VFB > 0.8V and the inductor current
goes slightly negative, then the MIC45205-1
automatically powers down most of the IC circuitry and
goes into a low-power mode.
Once the MIC45205-1 goes into discontinuous mode,
both DL and DH are low, which turns off the high-side
and low-side MOSFETs. The load current is supplied
by the output capacitors and VOUT drops. If the drop of
VOUT causes VFB to go below VREF, then all the circuits
will wake up into normal continuous mode. First, the
bias currents of most circuits reduced during the
discontinuous mode are restored, and then a tON pulse
is triggered before the drivers are turned on to avoid
any possible glitches. Finally, the high-side driver is
turned on. Figure 4-4 shows the control loop timing in
discontinuous mode.
VDH
FIGURE 4-3:
Response.
In order to meet the stability requirements, the
MIC45205 feedback voltage ripple should be in phase
with the inductor current ripple and are large enough to
be sensed by the gm amplifier and the error
comparator. The recommended feedback voltage
ripple is 20 mV~100 mV over full input voltage range. If
a low ESR output capacitor is selected, then the
feedback voltage ripple may be too small to be sensed
by the gm amplifier and the error comparator. Also, the
output voltage ripple and the feedback voltage ripple
are not necessarily in phase with the inductor current
ripple if the ESR of the output capacitor is very low. In
these cases, ripple injection is required to ensure
proper operation. Please refer to the Ripple Injection
subsection in the Application Information section for
more details about the ripple injection technique.
MIC45205 Load Transient
2017 - 2022 Microchip Technology Inc.
DS20005798B-page 15
MIC45205
IL CROSSES 0 AND VFB > 0.8V
DISCONTINUOUS MODE STARTS
MIC45205
VIN
VFB
