MIC2177
2.5A Synchronous Buck Regulator
Features
General Description
• 4.5V to 16.5V Input Voltage Range
• Dual Mode Operation for High Efficiency (up to
96%)
- PWM Mode for > 200 mA Load Current
- Skip Mode for < 200 mA Load Current
• 100 mΩ Internal Power MOSFETs at 12V Input
• 200 kHz Preset Switching Frequency
• Low Quiescent Current
- 1.0 mA in PWM Mode
- 500 μA in Skip Mode
- < 5 μA in Shutdown Mode
• 100% Duty Cycle for Low Dropout Operation
• Current-Mode Control
- Simplified Loop Compensation
- Superior Line Regulation
• Current Limit
• Thermal Shutdown
• Undervoltage Lockout
The MIC2177 is a 200 kHz synchronous buck
(step-down) switching regulator designed for
high-efficiency, battery-powered applications.
Applications
•
•
•
•
•
•
High-Efficiency, Battery-Powered Supplies
Buck (Step-Down) DC-to-DC Converters
Cellular Telephones
Laptop Computers
Handheld Instruments
Battery Charger
The MIC2177 operates from a 4.5V to 16.5V input and
features internal power MOSFETs that can supply up to
2.5A output current. It can operate with a maximum
duty cycle of 100% for use in low-dropout conditions. It
also features a shutdown mode that reduces quiescent
current to less than 5 μA.
The MIC2177 achieves high efficiency over a wide
output current range by switching between PWM and
skip mode. Operating mode is automatically selected
according to output conditions. Switching frequency is
preset to 200 kHz and can be synchronized to an
external clock signal of up to 300 kHz.
The MIC2177 uses current-mode control with internal
current sensing. Current-mode control provides
superior line regulation and makes the regulator control
loop easy to compensate. The output is protected with
pulse-by-pulse current limiting and thermal shutdown.
Undervoltage lockout turns the output off when the
input voltage is less than 4.5V.
The MIC2177 is packaged in a 20-pin wide power
SOIC package with an operating temperature range of
–40°C to +85°C.
Package Type
20-Pin Wide SOIC (WM)
VIN
1
VIN
2
19 BIAS
SW 3
18 SYNC
PGND 4
17 SGND
PGND 5
16 SGND
PGND 6
15 SGND
PGND 7
14 SGND
SW 8
13 COMP
VIN 9
12 FB
OUT 10
2020 Microchip Technology Inc.
20 EN
11 AUTO
DS20006298A-page 1
MIC2177
Typical Application
VIN
5.4V to 18V
C1
22μF
35V
U1
1,2,9
VIN
ENABLE
20
SHUTDOWN
18
11
2.2
nF
OUT
EN MIC2177-5.0
SW
SYNC
PGND
AUTO
FB
COMP SGND BIAS
R1
10k
CC
6.8nF
13
14–17
10
L1, 50μH
3,8
VOUT
5V/1A
C2
100μF
10V
D1
MBRS130L
4–7
12
19
C3
0.01μF
R1
10k
Functional Block Diagram
VIN
4.5V to 16.5V
CIN
VIN
UVLO,
Thermal
Shutdown
1
2
9
R1
VOUT = 1.245 (
+ 1)
R2
100m
P-channel
SW
ISENSE
Amp.
Output
Control
Logic
L1
3
D
EN
Enable
Shutdown
20
VOUT
8
3.3V
Regulator
100m
N-channel
COUT
PGND
4
BIAS
10k
5
ILIMIT
Comp.
19
0.01μF
6
7
internal
supply
voltage
Bold lines indicate
high current traces
PWM/
Skip-Mode
Select Logic
IMIN
Comp.
IMIN
Thrshld.
OUT
10
SYNC
18
CORRECTIVE RAMP
200kHz
Oscillator
R1
3.3V
Low Output
Comp.
FB
12
10μA
AUTO
Auto-Mode
PWM
R2
11
2.2nF
40mV
Skip-Mode
Comp.
RESET PULSE
R
Q
S
PWM
Comp.
Error
Amp.
COMP
RC
CC
VREF
1.245V
13
MIC2177 [Adjustable]
SGND
DS20006298A-page 2
14
15
16
17
2020 Microchip Technology Inc.
MIC2177
1.0
ELECTRICAL CHARACTERISTICS
Absolute Maximum Ratings †
Supply Voltage [100 ms Transient] (VIN) .................................................................................................................. +18V
Output Switch Voltage (VSW) ................................................................................................................................... +18V
Output Switch Current (ISW).................................................................................................................................... +6.0A
Enable, Output Sense Voltage (VEN, VOUT)............................................................................................................. +18V
Sync Pin Voltage (VSYNC) .......................................................................................................................................... +6V
Operating Ratings ‡
Supply Voltage (VIN) ............................................................................................................................... +4.5V to +16.5V
Junction Temperature (TJ)......................................................................................................................–40°C to +125°C
† 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. Specifications are for packaged product only.
‡ Notice: The device is not guaranteed to function outside its operating ratings.
ELECTRICAL CHARACTERISTICS
Electrical Characteristics: VIN = 7.0V; TA = +25°C, Bold values indicate –40°C ≤ TA ≤ +85°C; unless otherwise
specified.
Parameter
Input Supply Current
Bias Regulator Output
Voltage
Feedback Voltage
Output Voltage
Undervoltage Lockout
Feedback Bias Current
Error Amplifier Gain
Symbol
Min.
Typ.
Max.
Units
—
1.0
1.5
mA
PWM Mode, Output not Switching,
4.5V ≤ VIN ≤ 16.5V
—
500
650
μA
Skip Mode, Output not Switching,
4.5V ≤ VIN ≤ 16.5V
—
1
25
μA
VEN = 0V, 4.5V ≤ VIN ≤ 16.5V
VBIAS
3.10
3.30
3.40
V
VIN = 16.5V
VFB
1.22
1.245
1.27
V
MIC2177 [Adj.]: VOUT = 3.3V,
ILOAD = 0
3.20
3.14
3.3
3.40
3.46
V
V
MIC2177 [Adj.]: VOUT = 3.3V,
5V ≤ VIN ≤ 16V, 10 mA ≤ ILOAD ≤ 2A
4.85
5.0
5.15
V
MIC2177-5.0: ILOAD = 0
4.85
4.75
5.0
5.15
5.25
V
V
MIC2177-5.0:
6V ≤ VIN ≤ 16V, 10 mA ≤ ILOAD ≤ 2A
3.20
ISS
VOUT
Conditions
3.3
3.40
V
MIC2177-3.3: ILOAD = 0
3.20
3.14
3.3
3.40
3.46
V
V
MIC2177-3.3:
5V ≤ VIN ≤ 16V, 10 mA ≤ ILOAD ≤ 2A
VTH
—
4.25
4.35
V
Upper Threshold
VTL
3.9
4.15
—
V
Lower Threshold
—
60
150
nA
MIC2177 [Adj.]
—
20
40
μA
MIC2177-5.0, MIC2177-3.3
IFB
AVOL
5
18
30
V
0.6V ≤ VCOMP ≤ 0.8V
0.9
1.5
—
V
Upper Limit
Error Amplifier Output Swing
—
—
0.05
0.1
V
Lower Limit
Error Amplifier Output Current
—
15
25
38
μA
Source and Sink
Oscillator Frequency
fO
160
200
240
kHz
—
Maximum Duty Cycle
DMAX
100
—
—
%
VFB = 1.0V
tON(MIN)
—
300
400
ns
VFB = 1.5V
Maximum On-Time
2020 Microchip Technology Inc.
DS20006298A-page 3
MIC2177
ELECTRICAL CHARACTERISTICS
Electrical Characteristics: VIN = 7.0V; TA = +25°C, Bold values indicate –40°C ≤ TA ≤ +85°C; unless otherwise
specified.
Parameter
Symbol
Min.
Typ.
Max.
Units
Conditions
220
—
300
kHz
—
SYNC Frequency Range
—
SYNC Threshold
—
0.8
1.6
2.2
V
—
SYNC Minimum Pule Width
—
500
—
—
ns
—
SYNC Leakage
Current Limit
ISYNC
ILIM
–1
0.01
1
μA
VSYNC = 0V to 5.5V
3.8
4.7
5.7
A
PWM Mode, VIN = 12V
—
600
—
mA
Skip Mode
—
90
250
mΩ
High-Side Switch, VIN = 12V
Switch On-Resistance
RON
—
110
250
mΩ
Low-Side Switch, VIN = 12V
Output Switch Leakage
ISW
—
1
10
μA
VSW = 16.5V
Enable Threshold
—
0.8
1.6
2.2
V
—
Enable Leakage
IEN
–1
0.01
1
μA
VEN = 0V to 5.5V
AUTO Threshold
—
0.8
1.6
—
V
—
AUTO Source Current
—
7
11
15
μA
VFB = 1.5V, VAUTO < 0.8V
Minimum Switch Current for
PWM Operation
—
—
220
—
mA
VIN – VOUT = 0V
—
420
—
mA
VIN – VOUT = 3V
DS20006298A-page 4
2020 Microchip Technology Inc.
MIC2177
TEMPERATURE SPECIFICATIONS (Note 1)
Parameters
Symbol
Min.
Typ.
Max.
Units
TJ
–40
—
+125
°C
Conditions
Temperature Ranges
Junction Temperature Range
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.
2020 Microchip Technology Inc.
DS20006298A-page 5
MIC2177
2.0
TYPICAL PERFORMANCE CURVES
Note:
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.
5.030
REFERENCE VOLTAGE (V)
205
FREQUENCY (kHz)
200
195
190
185
180
175
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
FIGURE 2-1:
Temperature.
Oscillator Frequency vs.
MIC2177-5.0
5.020
5.010
5.000
4.990
4.980
4.970
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
FIGURE 2-4:
Temperature.
19.0
MIC2177 [adj.]
1.248
1.246
1.244
1.242
1.240
AMPLIFIER VOLTAGE GAIN
REFERENCE VOLTAGE (V)
1.252
1.250
1.238
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
FIGURE 2-2:
Temperature.
Reference Voltage vs.
18.0
17.5
17.0
16.5
FIGURE 2-5:
Temperature.
Error Amplifier Gain vs.
120
MIC2177-3.3
BIAS CURRENT (nA)
REFERENCE VOLTAGE (V)
18.5
16.0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
3.320
3.315
Reference Voltage vs.
3.310
3.305
3.300
3.295
3.290
3.285
3.280
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
FIGURE 2-3:
Temperature.
DS20006298A-page 6
Reference Voltage vs.
100
80
60
40
20
0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
FIGURE 2-6:
Feedback Input Bias
Current vs. Temperature.
2020 Microchip Technology Inc.
MIC2177
12
4.9
4.8
SUPPLY CURRENT (mA)
CURRENT LIMIT (A)
5.0
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4.0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
FIGURE 2-7:
Temperature.
Current Limit vs.
4
2
2
FIGURE 2-10:
4
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
PWM Mode Supply Current.
100
95
EFFICIENCY (%)
125°C
85°C
25°C
0°C
150
50
90
VIN = 5V
85
8V
80
12V
75
70
SKIP
PWM
65
2
4
FIGURE 2-8:
Resistance.
60
10
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
High-Side Switch On-
FIGURE 2-11:
100
1000 2500
OUTPUT CURRENT (mA)
3.3V Output Efficiency.
100
350
250
200
150
100
95
EFFICIENCY (%)
125°C
85°C
25°C
0°C
300
2
4
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
Low-Side Switch On-
2020 Microchip Technology Inc.
VIN = 6V
90
8V
85
12V
80
75
50
FIGURE 2-9:
Resistance.
6
100
200
0
8
0
250
0
OUTPUT
SWITCHING
10
70
10
FIGURE 2-12:
SKIP
PWM
100
1000 2500
OUTPUT CURRENT (mA)
5V Output Efficiency.
DS20006298A-page 7
MIC2177
3.0
PIN DESCRIPTIONS
The descriptions of the pins are listed in Table 3-1.
TABLE 3-1:
PIN FUNCTION TABLE
Pin Number
Pin
Name
1, 2, 9
VIN
Supply Input: Controller and switch supply. Unregulated supply input to internal
regulator, output switches, and control circuitry. Requires bypass capacitor to
PGND. All three pins must be connected to VIN.
3, 8
SW
Switch (Output): Internal power MOSFET switch output. Both pins must be
externally connected together.
4, 5, 6, 7
PGND
10
OUT
11
AUTO
12
FB
13
COMP
Compensation: Internal error amplifier output. Connect to capacitor or series RC
network to compensate the regulator control loop.
14, 15, 16, 17
SGND
Signal Ground: Ground connection of control section. Connect all pins to common
ground plane.
18
SYNC
Frequency Synchronization (Input): Optional clock input. Connect to external clock
signal to synchronize oscillator. Leading edge of signal above 1.7V terminates
switching cycle. Connect to SGND if not used.
19
BIAS
Bias Supply: Internal 3.3V bias supply output. Decouple with 0.01 μF bypass
capacitor and 10 kΩ to SGND. Do not apply any external load.
20
EN
DS20006298A-page 8
Description
Power Ground: Output stage ground connections. Connect all pins to a common
ground plane.
Output Voltage Sense (Input): Senses output voltage to determine minimum switch
current for PWM operation. Connect directly to OUT.
Automatic Mode: Connect 2.2 nF timing capacitor for automatic PWM/Skip mode
switching. Regulator operates exclusively in PWM mode when pin is pulled low.
Feedback (Input): Error amplifier inverting input. For adjustable output version,
connect FB to external resistive divider to set output voltage. For 3.3V and 5V fixed
output versions, connect FB directly to output.
Enable (Input): Logic high enables operation. Logic low shuts down regulator. Do
not allow pin to float.
2020 Microchip Technology Inc.
MIC2177
4.0
FUNCTIONAL DESCRIPTION
The MIC2177 is a synchronous buck regulator that
operates from an input voltage of 4.5V to 16.5V and
provides a regulated output voltage of 1.25V to 16.5V.
It has internal power MOSFETs that supply up to 2.5A
of load current and operates with up to 100% duty cycle
to allow low dropout operation. To optimize efficiency,
the MIC2177 operates in PWM and skip mode. Skip
mode provides the best efficiency when load current is
less than 200 mA, while PWM mode is more efficient at
higher current. A patented technique allows the
MIC2177 to automatically select the correct operating
mode as the load current changes.
During PWM operation, the MIC2177 uses current
mode control that provides superior line regulation and
makes the control loop easier to compensate. The
PWM switching frequency is set internally to 200 kHz
and can be synchronized to an external clock
frequency up to 300 kHz. Other features include a low
current shutdown mode, current limit, undervoltage
lockout, and thermal shutdown.
4.1
Switch Output
The switch output (SW) is a half H-bridge consisting of
a high side P-channel and low side N-channel power
MOSFET. These MOSFETs have a typical
on-resistance of 100 mΩ when the MIC2177 operates
from a 12V supply. Anti shoot through circuitry prevents
the P-channel and N-channel from turning on at the
same time.
4.2
Undervoltage Lockout
Undervoltage lockout (UVLO) turns off the output when
the input voltage (VIN) is too low to provide sufficient
gate drive for the output MOSFETs. It prevents the
output from turning on until VIN exceeds 4.3V. Once
operating, the output will not shut off until VIN drops
below 4.2V.
4.3
Thermal Shutdown
Thermal shutdown turns off the output when the
MIC2177 junction temperature exceeds the maximum
value for safe operation. After thermal shutdown
occurs, the output will not turn on until the junction
temperature drops approximately 10°C.
4.4
Shutdown Mode
The MIC2177 has a low-current shutdown mode that is
controlled by the enable input (EN). When a logic 0 is
applied to EN, the MIC2177 is in shutdown mode and
its quiescent current drops to less than 5 μA.
2020 Microchip Technology Inc.
4.5
Internal Bias Regulator
An internal 3.3V regulator provides power to the
MIC2177 control circuits. This internal supply is
brought out to the BIAS pin for bypassing by an
external 0.01 μF capacitor. Do not connect any external
load to the BIAS pin. It is not designed to provide an
external supply voltage.
4.6
Frequency Synchronization
The MIC2177 operates at a preset switching frequency
of 200 kHz. It can be synchronized to a higher
frequency by connecting an external clock to the SYNC
pin. The SYNC pin is a logic level input that
synchronizes the oscillator to the rising edge of an
external clock signal. It has a frequency range of
220 kHz to 300 kHz, and can operate with a minimum
pulse-width of 500 ns. If synchronization is not
required, connect SYNC to ground.
4.7
Low-Dropout Operation
Output regulation is maintained in PWM or skip mode
even when the difference between VIN and VOUT
decreases below 1V. As VIN - VOUT decreases, the duty
cycle increases until it reaches 100%. At this point, the
P-channel is kept on for several cycles at a time, and
the output stays in regulation until VIN - VOUT falls
below the dropout voltage (dropout voltage =
P-channel on resistance × load current).
4.8
PWM Mode Operation
Refer to PWM Mode Functional Block Diagram and
Timing Diagram which is a simplified block diagram of
the MIC2177 operating in PWM mode with its
associated waveforms.
When operating in PWM mode, the output P-channel
and N-channel MOSFETs are alternately switched on
at a constant frequency and variable duty cycle. A
switching period begins when the oscillator generates
a reset pulse. This pulse resets the RS latch which
turns on the P-channel and turns off the N-channel.
During this time, inductor current (IL1) increases and
energy is stored in the inductor. The current sense
amplifier (ISENSE Amp) measures the P-channel
drain-to-source voltage and outputs a voltage
proportional to IL1. The output of ISENSE Amp is added
to a saw tooth waveform (corrective ramp) generated
by the oscillator, creating a composite waveform
labeled ISENSE on the timing diagram. When ISENSE is
greater than the error amplifier output, the PWM
comparator will set the RS latch which turns off the
P-channel and turns on the N-channel. Energy is then
discharged from the inductor and IL1 decreases until
the next switching cycle begins. By varying the
P-channel on-time (duty cycle), the average inductor
current is adjusted to whatever value is required to
regulate the output voltage.
DS20006298A-page 9
MIC2177
The MIC2177 uses current-mode control to adjust the
duty cycle and regulate the output voltage.
Current-mode control has two signal loops that
determine the duty cycle. One is an outer loop that
senses the output voltage, and the other is a faster
inner loop that senses the inductor current. Signals
from these two loops control the duty cycle in the
following way: VOUT is fed back to the error amplifier
which compares the feedback voltage (VFB) to an
internal reference voltage (VREF). When VOUT is lower
than its nominal value, the error amplifier output
voltage increases. This voltage then intersects the
current sense waveform later in switching period which
increases the duty cycle and average inductor current.
If VOUT is higher than nominal, the error amplifier
output voltage decreases, reducing the duty cycle.
The PWM control loop is stabilized in two ways. First,
the inner signal loop is compensated by adding a
corrective ramp to the output of the current sense
amplifier. This allows the regulator to remain stable
when operating at greater than 50% duty cycle.
Second, a series resistor capacitor load is connected to
the error amplifier output (COMP pin). This places a
pole zero pair in the regulator control loop.
One more important item is synchronous rectification.
As mentioned earlier, the N-channel output MOSFET is
turned on after the P-channel turns off. When the
N-channel turns on, its on resistance is low enough to
create a short across the output diode. As a result,
inductor current flows through the N-channel and the
voltage drop across; it is significantly lower than a
diode forward voltage. This reduces power dissipation
and improves efficiency to greater than 95% under
certain operating conditions.
To prevent shoot through current, the output stage
employs break before make circuitry that provides
approximately 50 ns of delay from the time one
MOSFET turns off and the other turns on. As a result,
inductor current briefly flows through the output diode
during this transition.
4.9
Skip Mode Operation
Refer to Skip Mode Functional Block Diagram and
Timing Diagram which is a simplified block diagram of
the MIC2177 operating in skip mode and its associated
waveforms.
Skip-mode operation turns on the output P-channel at
a frequency and duty cycle that is a function of VIN,
VOUT, and the output inductor value. While in skip
mode, the N-channel is kept off to optimize efficiency
by reducing gate charge dissipation. VOUT is regulated
by skipping switching cycles that turn on the P-channel.
To begin analyzing MIC2177 skip mode operation,
assume the skip mode comparator output is high and
the latch output has been reset to a logic 1. This turns
on the P-channel and causes IL1 to increase linearly
until it reaches a current limit of 600 mA. When IL1
DS20006298A-page 10
reaches this value, the current limit comparator sets the
RS latch output to logic 0, turning off the P-channel.
The output switch voltage (VSW) then swings from VIN
to 0.4V below ground, and IL1 flows through the
Schottky diode. L1 discharges its energy to the output
and IL1 decreases to zero. When IL1 = 0, VSW swings
from –0.4V to VOUT, and this triggers a one-shot that
resets the RS latch. Resetting the RS latch turns on the
P-channel, which begins another switching cycle.
The skip mode comparator regulates VOUT by
controlling when the MIC2177 skips cycles. It
compares VFB to VREF and has 10 mV of hysteresis to
prevent oscillations in the control loop. When VFB is
less than VREF – 5 mV, the comparator output is logic
1, allowing the P-channel to turn on. Conversely, when
VFB is greater than VREF + 5 mV, the P-channel is
turned off.
Note that this is a self-oscillating topology which
explains why the switching frequency and duty cycle
are a function of VIN, VOUT, and the value of L1. It has
the unique feature (for a pulse skipping regulator) of
supplying the same value of maximum load current for
any value of VIN, VOUT, or L1. This allows the MIC2177
to always supply up to 300 mA of load current (ILOAD)
when operating in skip mode.
4.10
Changing from PWM to Skip Mode
Refer to the Functional Block Diagram for circuits
described in the following sections.
The MIC2177 automatically changes from PWM to skip
mode operation when ILOAD drops below a minimum
value. IMIN is determined indirectly by detecting when
the peak inductor current (IL(peak)) is less than 420 mA.
This is done by the minimum current comparator which
detects if the output P-Channel current equals 420 mA
during each switching cycle. If it does not, the
PWM/skip mode select logic places the MIC2177 into
skip mode operation.
The value of IMIN that corresponds to IL1(peak) =
420 mA is given by the following equation:
EQUATION 4-1:
420mA – I L1
I MIN = ----------------------------------2
Where:
∆IL1 =
Inductor Ripple Current
Equation 4-1 shows IMIN varies as a function of ∆IL.
Therefore, the user must select an inductor value that
results in IMIN = 200 mA when IL(peak) = 420 mA. The
formulas for calculating the correct inductor value are
given in Section 5.0, Applications Information. Note
2020 Microchip Technology Inc.
MIC2177
that ∆IL varies as a function of input voltage, and this
also causes IMIN to vary. In applications where the input
voltage changes by a factor of two, IMIN will typically
vary from 130 mA to 250 mA.
During low dropout operation, the minimum current
threshold circuit reduces the minimum value of IL1(peak)
for PWM operation. This compensates for ∆IL1
decreasing to almost zero when the difference between
VIN and VOUT is very low.
4.11
Switching from Skip to PWM Mode
The MIC2177 will automatically change from skip to
PWM mode when ILOAD exceeds 300 mA. During skip
mode operation, it can supply up to 300 mA, and when
ILOAD exceeds this limit, VOUT will fall below its nominal
value. At this point, the MIC2177 begins operating in
PWM mode. Note that the maximum value of ILOAD for
skip mode is greater than the minimum value required
for PWM mode. This current hysteresis prevents the
MIC2177 from toggling between modes when ILOAD is
in the range of 100 mA to 300 mA.
The low output comparator determines when VOUT is
low enough for the regulator to change operating
modes. It detects when the feedback voltage is 3%
below nominal, and pulls the AUTO pin to ground.
When AUTO is less than 1.6V, the PWM/Skip-mode
select logic places the MIC2177 into PWM operation.
The external 2.2 nF capacitor connected to AUTO is
charged by a 10 μA current source after the regulator
begins operating in PWM mode. As a result, AUTO
stays below 1.6V for several switching cycles after
PWM operation begins, forcing the MIC2177 to remain
in PWM mode during this transition.
4.12
External PWM Mode Selection
The MIC2177 can be forced to operate in only PWM
mode by connecting AUTO to ground. This prevents
skip mode operation in applications that are sensitive to
switching noise.
2020 Microchip Technology Inc.
DS20006298A-page 11
MIC2177
PWM Mode Functional Block Diagram and Timing Diagram
VIN
4.5V to 16.5V
CIN
VIN
1
2
9
R1
VOUT = 1.245 (
+ 1)
R2
100m
P-channel
SW
ISENSE
Amp.
L1
3
VOUT
8
IL1
D
100m
N-channel
COUT
PGND
4
5
6
7
Corrective
Ramp
Stop
SYNC
200kHz
Oscillator
18
R1
Reset
Pulse
FB
12
R2
R
Q
S
PWM
Comp.
Error
Amp.
COMP
CC
RC
13
VREF 1.245V
MIC2177 [Adjustable] PWM-Mode Signal Path
SGND
14
15
16
17
VSW
Reset
Pulse
I L1
I LOAD
IL1
Error Amp.
Output
I SENSE
DS20006298A-page 12
2020 Microchip Technology Inc.
MIC2177
Skip Mode Functional Block Diagram and Timing Diagram
VIN
4.5V to 16.5V
CIN
VIN
1
2
9
Output Control Logic
S
Q
R1
VOUT = 1.245 (
+ 1)
R2
100m
P-channel
R
One
Shot
ISENSE
Amp.
SW
L1
3
VOUT
8
IL1
D
COUT
PGND
4
5
ILIMIT
Comp.
6
7
ILIMIT
Thresh.
Voltage
R1
Skip-Mode
Comp.
FB
12
R2
VREF 1.245V
MIC2177 [Adjustable] Skip-Mode Signal Pat
SGND
VSW
14
15
16
17
VIN
VOUT
0
One-Shot
Pulse
I LIM
I L1
0
VREF + 5mV
VFB
VREF – 5mV
2020 Microchip Technology Inc.
DS20006298A-page 13
MIC2177
5.0
APPLICATIONS INFORMATION
5.1
Feedback Resistor Selection
(Adjustable Version)
The output voltage is configured by connecting an
external resistive divider to the FB pin as shown in
Functional Block Diagram. The ratio of R1 to R2
determines the output voltage. To optimize efficiency
during low output current operation, R2 should not be
less than 20 kΩ. However, to prevent feedback error
due to input bias current at the FB pin, R2 should not
be greater than 100 kΩ. After selecting R2, calculate
R1 using the following formula:
Inductor Selection
The inductor must be at least a minimum value in order
for the MIC2177 to change from PWM to skip mode at
the correct value of output current. This minimum value
ensures the inductor ripple current never exceeds
600 mA, and is calculated using the following formula:
EQUATION 5-3:
V OUT
- 8.3 H
L MIN = V OUT 1 – -----------------------
V
IN MAX
Where:
VIN(MAX) =
EQUATION 5-1:
V OUT
R1 = R2 ----------------- –1
1.245V
5.2
5.3
Input Capacitor Selection
The input capacitor is selected for its RMS current and
voltage rating and should be a low ESR (equivalent
series resistance) electrolytic or tantalum capacitor. As
a rule of thumb, the voltage rating for a tantalum
capacitor should be twice the value of VIN, and the
voltage rating for an electrolytic should be 40% higher
than VIN. The RMS current rating must be equal or
greater than the maximum RMS input ripple current. A
simple, worst case formula for calculating this RMS
current is:
Maximum Input Voltage
In general, a value at least 20% greater than LMIN
should be selected because inductor values have a
tolerance of ±20%.
Two other parameters to consider in selecting an
inductor are winding resistance and peak current
rating. The inductor must have a saturation current
rating equal or greater than the peak inductor current.
Otherwise, the inductor may saturate, causing
excessive current in the output switch. Also, the
inductor’s core loss may increase significantly. Both of
these effects will degrade efficiency. The formula for
peak inductor current is:
EQUATION 5-4:
I L PEAK = I LOAD MAX + 300mA
EQUATION 5-2:
I RMS MAX
I LOAD MAX
= -----------------------------2
To maximize efficiency, the inductor’s resistance must
be less than the output switch on-resistance (preferably
50 mΩ or less).
5.4
Tantalum capacitors are a better choice for applications
that require the most compact layout or operation
below 0°C. The input capacitor must be located very
close to the VIN pin (within 0.2 inches, 5 mm). Also
place a 0.1 μF ceramic bypass capacitor as close as
possible to VIN.
DS20006298A-page 14
Output Capacitor Selection
Select an output capacitor that has a low value of ESR.
This parameter determines a regulator’s output ripple
voltage (VRIPPLE) which is generated by ∆IL × ESR. As
mentioned in Section 5.3, Inductor Selection the
maximum value for ∆IL is 600 mA.
Therefore, the maximum value of ESR is:
2020 Microchip Technology Inc.
MIC2177
5.7
EQUATION 5-5:
ESR MAX
V RIPPLE
= --------------------600mA
Where:
VRIPPLE <
1% of VOUT
Typically, capacitors in the range of 100 μF to 220 μF
have ESR less than this maximum value. The output
capacitor can be either a low ESR electrolytic or
tantalum capacitor, but tantalum is a better choice for
compact layout and operation at temperatures below
0°C. The voltage rating of a tantalum capacitor must be
2 × VOUT, and the voltage rating of an electrolytic must
be 1.4 × VOUT.
5.5
Output Diode Selection
In PWM operation, inductor current flows through the
output diode approximately 50 ns during the dead time
when one output MOSFET turns off and the other turns
on. In skip mode, the inductor current flows through the
diode during the entire P-channel off time. The correct
diode for both of these conditions is a 1A diode with a
reverse voltage rating greater than VIN. It must be a
Schottky or ultra-fast recovery diode (tR
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