MIC2179
1.5A Synchronous Buck Regulator
Features
General Description
• Input Voltage Range: +4.5V to +16.5V
• Dual-Mode Operation for High Efficiency (up to
96%):
- PWM Mode for > 150 mA Load Current
- Skip Mode for < 150 mA Load Current
• 150 mΩ Internal Power MOSFETs at 12V Input
• 200 kHz Preset Switching Frequency
• Low Quiescent Current
- 1.0 mA in PWM Mode
- 600 μA in Skip Mode
- < 5 μA in Shutdown Mode
• Current-Mode Control
- Simplified Loop Compensation
- Superior Line Regulation
• 100% Duty Cycle for Low Dropout Operation
• Current Limit
• Thermal Shutdown
• Undervoltage Lockout
The MIC2179 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/DC Converters
Laptop Computers
Cellular Telephones
Handheld Instruments
Battery Chargers
The MIC2179 operates from a 4.5V to 16.5V input and
features internal power MOSFETs that can supply up to
1.5A 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 MIC2179 achieves high efficiency over a wide
output current range by operating in either PWM or skip
mode. The operating mode is externally selected,
typically by an intelligent system, which chooses the
appropriate mode according to operating conditions,
efficiency, and noise requirements. The switching
frequency is preset to 200 kHz and can be
synchronized to an external clock signal of up to
300 kHz.
The MIC2179 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 MIC2179 is packaged in a 20-lead SSOP package
with an operating temperature range of -40°C to +85°C.
Package Type
MIC2179
20-Lead SSOP (SM)
(Top View)
PGND 1
20 PGND
PGND 2
19 PGND
SW 3
18 NC
NC 4
17 VIN
PWM 5
16 VIN
PWRGD 6
FB 7
2021 Microchip Technology Inc.
15 EN
14 BIAS
COMP 8
13 SYNC
SGND 9
12 SGND
SGND 10
11 SGND
DS20006284B-page 1
MIC2179
Typical Application Circuit
VIN
5.4V to 16.5V
C1
10μF
20V
U1
R1
20k
15
Output Good
Output Low
Skip Mode
PWM Mode
6
5
13
16,17
L1
22μH
VIN
EN
3,4
SW
PWRGD
PWM
SYNC
COMP
8
1,2,
19,20
MIC
2179-3.3 PGND
C2
100μF
6.3V
7
FB
SGND
VOUT
3.3V/600mA
D1
MBRM120
BIAS
9–12
14
C3
0.01μF
C4
6.8nF
R5
4.02k
Pins 4 and 18 are not connected.
Pins 3 and 4 can be connected
together for a low-impedance
connection.
Functional Block Diagram
V IN
4.5V to 16.5V
100μF
VIN
UVLO,
Thermal
Shutdown
Enable
Shutdown
EN
15
16
17
V OUT
150m Ω
P-channel
I SENSE
Amp.
Output
Control
Logic
3.3V
Regulator
1.245
D
150m Ω
N-channel
0.01μF
C OUT
PGND
1
Internal
Supply Voltage
PWM
5
2
I LIMIT
Comp.
SYNC
13
*
19
* Connect
S GND to P GND
20
PWM/
Skip-Mode
Select
I LIMIT
Thresh.
Voltage
Bold lines indicate
high current traces
Corrective
Ramp
Stop
V OUT
3
14
Skip Mode
PWM Mode
200kHz
Oscillator
R1
Skip-Mode
Comp.
Reset
Pulse
FB
R
7
Q
S
R2
V IN
Power Good
Comp.
PWM
Comp.
RC
1
L
SW
BIAS
R3
4.02kΩ
R1
R2
20kΩ
PWRGD
Output Good
6
COMP
8
CC
1.13V
V REF 1.245V
MIC2179 [Adjustable]
SGND
DS20006284B-page 2
9
10
11
12
2021 Microchip Technology Inc.
MIC2179
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, PWM Control Voltage (VEN, VPWM) .............................................................................................................+18V
Sync Voltage (VSYNC) .................................................................................................................................................+6V
Operating Ratings ††
Supply Voltage (VIN) ............................................................................................................................... +4.5V to +16.5V
† 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 = 7.0V; TA = +25°C, bold indicates –40°C ≤ TA ≤ +85°C; unless noted. Devices are
ESD sensitive. Handling precautions recommended.
Parameter
Input Supply Current
Bias Regulator Output Voltage
Feedback Voltage
Output Voltage
Undervoltage Lockout
Feedback Bias Current
Error Amplifier Gain
Error Amplifier Output Swing
2021 Microchip Technology Inc.
Sym.
Min.
Typ.
Max.
Units
Conditions
—
1.0
1.5
mA
PWM mode, output not switching,
4.5V ≤ VIN ≤ 16.5V
—
600
750
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
3.4
V
VIN = 16.5V
VFB
1.22
1.245
1.27
V
MIC2179 [adj.]: VOUT = 3.3V,
ILOAD = 0A
3.20
3.3
3.40
V
3.14
—
3.46
V
4.85
5.0
5.15
V
MIC2179-5.0: ILOAD = 0A
4.85
5.0
5.15
V
4.75
—
5.25
V
MIC2179-5.0: 6V ≤ VIN ≤ 16V,
10 mA ≤ ILOAD ≤ 1A
3.20
3.3
3.40
V
MIC2179-3.3: ILOAD = 0A
3.20
3.3
3.40
V
3.14
—
3.46
V
MIC2179-3.3: 5V ≤ VIN ≤ 16V,
10 mA ≤ ILOAD ≤ 1A
VTH
—
4.25
4.35
V
Upper threshold
VTL
3.90
4.15
—
V
Lower threshold
—
60
150
nA
MIC2179 [adj.]
—
20
40
µA
MIC2179-5.0, MIC2179-3.3
15
18
20
—
0.6V ≤ VCOMP ≤ 0.8V
0.9
15
—
—
0.05
0.1
ISS
VOUT
IFB
AVOL
—
V
MIC2179 [adj.]: VOUT = 3.3V,
5V ≤ VIN ≤ 16V,
10 mA ≤ ILOAD ≤ 1A
Upper Limit
Lower Limit
DS20006284B-page 3
MIC2179
ELECTRICAL CHARACTERISTICS (CONTINUED)
Electrical Characteristics: VIN = 7.0V; TA = +25°C, bold indicates –40°C ≤ TA ≤ +85°C; unless noted. Devices are
ESD sensitive. Handling precautions recommended.
Parameter
Sym.
Min.
Typ.
Max.
Units
Error Amplifier Output Current
—
15
25
35
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
SYNC Frequency Range
—
220
—
300
kHz
—
SYNC Threshold
—
0.8
1.6
2.2
V
—
SYNC Minimum Pulse Width
—
500
—
—
ns
—
ISYNC
–1
0.01
1
µA
VSYNC = 0V to 5.5V
3.4
4.3
5.5
A
PWM mode, VIN = 12V
—
600
—
mA
—
160
350
—
140
350
Minimum On-Time
SYNC Leakage
Conditions
Current Limit
ILIM
Switch On-Resistance
RON
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
PWM Threshold
—
0.6
1.1
1.4
V
—
PWM Leakage
IPWM
–1
0.01
1
µA
VPWM = 0V to 5.5V
1.09
1.13
1.17
4.33
4.54
4.75
mΩ
Skip mode
High-side switch, VIN = 12V
Low-side switch, VIN = 12V
MIC2179 [adj.]: measured at FB
pin
PWRGD Threshold
—
V
2.87
3.00
3.13
PWRGD Output Low
—
—
0.25
0.4
V
ISINK = 0.5 mA
PWRGD Off Leakage
—
—
0.01
1
µA
VPWRGD = 5.5V
MIC2179-5.0: measured at FB pin
MIC2179-3.3: measured at FB pin
TEMPERATURE SPECIFICATIONS
Parameters
Sym.
Min.
Typ.
Max.
Units
TJ
–40
—
+125
°C
Conditions
Temperature Ranges
Operating Junction Temperature
Range
Note 1:
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.
DS20006284B-page 4
2021 Microchip Technology Inc.
MIC2179
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.
205
REFERENCE VOLTAGE (V)
5.030
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.
MIC2179-5.0
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
MIC2179 [adj.]
1.248
1.246
1.244
1.242
1.240
AMPLIFIER VOLTAGE GAIN
REFERENCE VOLTAGE (V)
1.252
1.250
Reference Voltage vs.
17.5
17.0
16.5
Error-Amplifier Gain vs.
120
MIC2179-3.3
BIAS CURRENT (nA)
REFERENCE VOLTAGE (V)
18.0
FIGURE 2-5:
Temperature.
3.320
3.315
18.5
16.0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
1.238
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
FIGURE 2-2:
Temperature.
Reference Voltage vs.
3.310
3.305
3.300
3.295
3.290
100
80
60
40
20
3.285
3.280
-60 -30 0
30 60 90 120 150
TEMPERATURE (°C)
FIGURE 2-3:
Temperature.
Reference Voltage vs.
2021 Microchip Technology Inc.
0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
FIGURE 2-6:
Feedback Input Bias
Current vs. Temperature.
DS20006284B-page 5
MIC2179
12
5.3
5.1
4.9
4.7
4.5
4.3
4.1
3.9
3.7
3.5
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
FIGURE 2-7:
Temperature.
Current Limit vs.
SUPPLY CURRENT (mA)
CURRENT LIMIT (A)
5.5
6
4
2
2
4
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
FIGURE 2-10:
PWM-Mode Supply Current.
95
125°C
85°C
25°C
0°C
300
250
90
200
150
100
2
4
FIGURE 2-8:
On-Resistance.
6 8 10 12 14 16 18
,138792/7$*( 9
High-Side Switch
8.4V
S kip
80
75
8.4V
P WM
70
65
50
5.4V
P WM
85
EFFICIENCY (%)
215(6,67$1&( P
8
0
350
0
OUTPUT
SWITCHING
10
60
10
FIGURE 2-11:
Efficiency.
5.4V
S kip
100
OUTPUT CURRENT (mA)
600
Skip-Mode and PWM-Mode
215(6,67$1&( P
400
125°C
85°C
25°C
0°C
350
300
250
200
150
100
50
0
2
4
FIGURE 2-9:
On-Resistance.
DS20006284B-page 6
6 8 10 12 14 16 18
,138792/7$*( 9
Low-Side Switch
2021 Microchip Technology Inc.
MIC2179
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, 19, 20
PGND
Description
Power ground: Connect all pins to central ground point.
3
SW
Switch (output): Internal power MOSFET output switches.
4, 18
NC
Not internally connected.
5
PWM
PWM/Skip Mode Control (input): Logic-level input. Controls regulator operating mode.
Logic low enables PWM mode. Logic high enables skip mode. Do not allow pin to float.
6
PWRGD
Error flag (output): Open-drain output. Active-low when FB input is 10% below the reference voltage (VREF).
7
FB
Feedback (input): Connect to output voltage divider resistors.
8
COMP
Compensation: Output of internal error amplifier. Connect capacitor or series RC network to compensate the regulator control loop.
9, 10, 11, 12
SGND
Signal ground: Connect all pins to ground, PGND.
13
SYNC
Frequency synchronization (input): Optional. Connect an external clock signal to synchronize the oscillator. Leading edge of signal above 1.7V terminates switching cycle.
Connect to SGND if not used.
14
BIAS
Internal 3.3V bias supply: Decouple with 0.01 µF bypass capacitor to SGND. Do not
apply any external load.
15
EN
Enable (input): Logic high enables operation. Logic low shuts down regulator. Do not
allow pin to float.
16, 17
VIN
Supply voltage (input): Requires bypass capacitor to PGND. Both pins must be connected to VIN.
2021 Microchip Technology Inc.
DS20006284B-page 7
MIC2179
4.0
FUNCTIONAL DESCRIPTION
The MIC2179 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 1.5A
load current and operates with up to 100% duty cycle
to allow low-dropout operation. To optimize efficiency,
the MIC2179 operates in PWM and skip mode. Skip
mode provides the best efficiency when load current is
less than 150 mA, while PWM mode is more efficient at
higher current. PWM or skip mode operation is
selected externally, allowing an intelligent system (i.e.
microprocessor controlled) to select the correct
operating mode for efficiency and noise requirements.
During PWM operation, the MIC2179 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. See the
following sections for more detail.
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 150 mΩ when the MIC2179 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
Current Limit
The MIC2179 uses pulse-by-pulse current limiting to
protect the output. During each switching period, a
current limit comparator detects if the P-channel
current exceeds 4.3A. When it does, the P-channel is
turned off until the next switching period begins.
4.3
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.
4.4
Thermal Shutdown
Thermal shutdown turns off the output when the
MIC2179 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.
DS20006284B-page 8
4.5
Shutdown Mode
The MIC2179 has a low-current shutdown mode that is
controlled by the enable input (EN). When a logic 0 is
applied to EN, the MIC2179 is in shutdown mode, and
its quiescent current drops to less than 5 μA.
4.6
Internal Bias Regulator
An internal 3.3V regulator provides power to the
MIC2179 control circuits. This internal supply is
brought out to the BIAS pin for bypassing by an
external 0.01 μF capacitor. Do not connect an external
load to the BIAS pin. It is not designed to provide an
external supply voltage.
4.7
Frequency Synchronization
The MIC2179 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.
4.8
Power Good Flag
The power good flag (PWRGD) is an error flag that
alerts a system when the output is not in regulation.
When the output voltage is 10% below its nominal
value, PWRGD is logic low, signaling that VOUT is too
low. PWRGD is an open-drain output that can sink
1 mA from a pull-up resistor connected to VIN.
4.9
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).
4.10
PWM-Mode Operation
Refer to the PWM Mode Functional Diagram which is a
simplified block diagram of the MIC2179 operating in
PWM mode and 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
2021 Microchip Technology Inc.
MIC2179
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 sawtooth 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.
The MIC2179 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 the 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.
2021 Microchip Technology Inc.
4.11
Skip Mode Operation
Refer to the Skip Mode Functional Diagram which is a
simplified block diagram of the MIC2179 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 MIC2179 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
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, and this begins another switching cycle.
The skip-mode comparator regulates VOUT by
controlling when the MIC2179 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 that 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 MIC2179
to always supply up to 300 mA of load current when
operating in skip mode.
4.12
Selecting PWM- or Skip-Mode
Operation
PWM or skip mode operation is selected by an external
logic signal applied to the PWM pin. A logic low places
the MIC2179 into PWM mode, and logic high places it
into skip mode. Skip mode operation provides the best
efficiency when load current is less than 150 mA, and
PWM operation is more efficient at higher currents.
The MIC2179 was designed to be used in intelligent
systems that determine when it should operate in PWM
or skip mode. This makes the MIC2179 ideal for
applications where a regulator must guarantee low
noise operation when supplying light load currents,
such as cellular telephone, audio, and multimedia
circuits.
DS20006284B-page 9
MIC2179
current of approximately 300 mA, so the output will
drop out of regulation when load current exceeds this
limit. To prevent this from occurring, the MIC2179
should change from skip to PWM mode when load
current exceeds 200 mA.
There are two important items to be aware of when
selecting PWM or skip mode. First, the MIC2179 can
start-up only in PWM mode, and therefore requires a
logic low at PWM during start-up. Second, in skip
mode, the MIC2179 will supply a maximum load
PWM Mode Functional Diagram
VIN
4.5V to 16.5V
CIN
VIN
16
17
VOUT = 1.245
150m
P-channel
IS E N S E
Amp.
( R1
+ 1)
R2
L1
SW
VOUT
3
IL1
D
150m
N-channel
COUT
PGND
1
2
19
20
Corrective
Ramp
Stop
S Y NC
13
200kHz
Oscillator
R1
Reset
Pulse
FB
7
R2
R
Q
S
PWM
Comp.
Error
Amp.
COMP
CC
RC
8
VR E F1.245V
MIC2179 [Adjustable] PWM-Mode Signal Path
SGND
9
10
11
12
VS W
Reset
Pulse
IL1
ILOAD
¨IL1
Error Amp.
Output
IS E N S E
DS20006284B-page 10
2021 Microchip Technology Inc.
MIC2179
Skip Mode Functional Diagram
VIN
4.5V to 16.5V
CIN
VIN
16
17
Output Control Logic
S
Q
VOUT = 1.245
150m
P-channel
R
One
Shot
IS E N S E
Amp.
( R1
+ 1)
R2
L1
SW
VOUT
3
IL1
D
COUT
PGND
1
2
ILIMIT
Comp.
19
20
ILIMIT
Thresh.
Voltage
R1
Skip-Mode
Comp.
FB
7
R2
VR E F1.245V
MIC2179 [Adjustable] Skip-Mode Signal Path
SGND
VS W
9
10
11
12
VIN
VOUT
0
One-Shot
Pulse
ILIM
IL1
0
VR E F + 5mV
VF B
VR E F – 5mV
2021 Microchip Technology Inc.
DS20006284B-page 11
MIC2179
5.0
APPLICATION INFORMATION
5.1
Feedback Resistor Selection
(Adjustable Version)
The output voltage is programmed by connecting an
external resistive divider to the FB pin as shown in the
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 with the following formula:
EQUATION 5-1:
V OUT
R1 = R2 ----------------- – 1
1.245V
EQUATION 5-3:
L MIN = V OUT 3.0H/V
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 peak 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:
5.2
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:
EQUATION 5-2:
I L MAX
I L PEAK = I LOAD MAX + ---------------------2
Where:
V OUT
1
I L MAX = V OUT 1 – ---------------------- ----------V IN MAX L f
To maximize efficiency, the inductor’s resistance must
be less than the output switch on-resistance
(preferably, 50 mΩ or less).
5.4
I RMS MAX
I LOAD MAX
= ----------------------------2
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 in, 5 mm). Also, place a
0.1 μF ceramic bypass capacitor as close as possible
to VIN.
5.3
Inductor Selection
The MIC2179 is a current-mode controller with internal
slope compensation. As a result, the inductor must be
at least a minimum value to prevent subharmonic
oscillations. This minimum value is calculated by the
following formula:
DS20006284B-page 12
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 ΔIL x ESR.
Therefore, ESR must be equal or less than a maximum
value calculated for a specified VRIPPLE (typically less
than 1% of the output voltage) and ΔIL(MAX):
EQUATION 5-5:
V RIPPLE
ESR MAX = ---------------------I L MAX
Typically, capacitors in the range of 100 µF to 220 μF
have ESR less than this maximum value. The output
capacitor can be a low ESR electrolytic or tantalum
capacitor, but tantalum is a better choice for compact
2021 Microchip Technology Inc.
MIC2179
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 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 ultrafast-recovery diode (tR
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