Micrel MIC2177-3.3BWM 2.5a synchronous buck regulator Datasheet

MIC2177
Micrel
MIC2177
2.5A Synchronous Buck Regulator
General Description
Features
The Micrel MIC2177 is a 200kHz synchronous buck (stepdown) switching regulator designed for high-efficiency, battery-powered applications.
• 4.5V to 16.5V input voltage range
• Dual-mode operation for high efficiency (up to 96%)
PWM mode for > 200mA load current
Skip mode for < 200mA load current
• 100mΩ internal power MOSFETs at 12V input
• 200kHz preset switching frequency
• Low quiescent current
1.0mA in PWM mode
500µA in skip mode
< 5µA 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 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
5µA.
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 200kHz and can
be synchronized to an external clock signal of up to 300kHz.
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.
Applications
•
•
•
•
•
•
The MIC2177 is packaged in a 20-lead wide power SO
package with an operating temperature range of –40°C to
+85°C.
See the MIC2178 for externally selected PWM or skip-mode
operation.
High-efficiency, battery-powered supplies
Buck (step-down) dc-to-dc converters
Cellular telephones
Laptop computers
Hand-held instruments
Battery Charger
Typical Application
VIN
6V to 16.5V
C2
22µF
35V
U1
ON
ENABLE 20
OFF
1,2,9
100
VIN
EN
OUT
MIC2177-5.0 SW
AUTO
MODE
Mode
PWM
MODE
C6
2.2nF
April 1999
11
18
AUTO
PGND
SYNC
COMP SGND
R7
15k
C5
10nF
13
14–17
FB
BIAS
5V Output
Efficiency
10
3,8
4–7
L1, 33µH
VOUT
5V/2.5A
D1
MBRS140
C7
220µF
10V
12
19
C3
220µF
10V
95
EFFICIENCY (%)
C1
22µF
35V
90
85
80
75
C4
0.01µF
70
10
1
VIN = 6V
SKIP
PWM
100
1000 2500
OUTPUT CURRENT (mA)
MIC2177
MIC2177
Micrel
Ordering Information
Part
Number
Output
Voltage
Switching
Frequency
Temperature
Range
Package
MIC2177-3.3BWM
3.3V
200kHz
–40°C to +85°C
20-lead wide SOP
MIC2177-5.0BWM
5.0V
200kHz
–40°C to +85°C
20-lead wide SOP
MIC2177BWM
adj.
200kHz
–40°C to +85°C
20-lead wide SOP
Pin Configuration
VIN 1
20 EN
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
11 AUTO
OUT 10
20-Lead Wide SOP
Pin Description
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
Output Voltage Sense (Input): Senses output voltage to determine minimum
switch current for PWM operation. Connect directly to VOUT.
11
AUTO
Automatic Mode: Connect 2.2nF timing capacitor for automatic PWM-/skipmode switching. Regulator operates exclusively in PWM mode when pin is
pulled low.
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 0.01µF
bypass capacitor to SGND. Do not apply any external load.
20
EN
Enable (Input): Logic high enables operation. Logic low shuts down
regulator. Do not allow pin to float.
MIC2177
Pin Function
Power Ground: Output stage ground connections. Connect all pins to a
common ground plane.
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.
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April 1999
MIC2177
Micrel
Absolute Maximum Ratings
Operating Ratings
Supply Voltage [100ms transient] (VIN) ......................... 18V
Output Switch Voltage (VSW) ........................................ 18V
Output Switch Current (ISW) ......................................... 6.0A
Enable, Output-Sense Voltage (VEN, VOUT) ................. 18V
Sync Voltage (VSYNC) ..................................................... 6V
Supply Voltage (VIN) ..................................... 4.5V to 16.5V
Junction Temperature Range (TJ) ........... –40°C to +125°C
Electrical Characteristics
VIN = 7.0V; TA = 25°C, bold indicates –40°C ≤ TA ≤ 85°C; unless noted.
Symbol
Parameter
Condition
ISS
Input Supply Current
Min
Typ
Max
Units
PWM mode, output not switching,
4.5V ≤ VIN ≤ 16.5V
1.0
1.5
mA
skip mode, output not switching,
4.5V ≤ VIN ≤ 16.5V
500
650
µA
1
25
µA
VEN = 0V, 4.5V ≤ VIN ≤ 16.5V
VBIAS
Bias Regulator Output Voltage
VIN = 16.5V
3.10
3.30
3.4
V
VFB
Feedback Voltage
MIC2177 [adj.]: VOUT = 3.3V, ILOAD = 0
1.22
1.245
1.27
V
VOUT
Output Voltage
MIC2177 [adj.]: VOUT = 3.3V,
5V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 2A
3.20
3.14
3.3
3.40
3.46
V
V
MIC2177-5.0: ILOAD = 0
4.85
5.0
5.15
V
MIC2177-5.0:
6V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 2A
4.85
4.75
5.0
5.15
5.25
V
MIC2177-3.3: ILOAD = 0
3.20
3.3
3.40
V
MIC2177-3.3:
5V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 2A
3.20
3.14
3.3
3.40
3.46
V
V
4.25
4.35
V
VTH
Undervoltage Lockout
VTL
IFB
AVOL
upper threshold
lower threshold
Feedback Bias Current
3.9
4.15
MIC2177 [adj.]
60
150
nA
MIC2177-5.0, MIC2177-3.3
20
40
µA
20
Error Amplifier Gain
0.6V ≤ VCOMP ≤ 0.8V
15
18
Error Amplifier Output Swing
upper limit
0.9
1.5
lower limit
Error Amplifier Output Current
V
source and sink
fO
Oscillator Frequency
DMAX
Maximum Duty Cycle
VFB = 1.0V
tON min
Minimum On-Time
VFB = 1.5V
V
0.05
0.1
V
15
25
35
µA
160
200
240
kHz
100
%
300
SYNC Frequency Range
220
SYNC Threshold
0.8
SYNC Minimum Pulse Width
500
1.6
400
ns
300
kHz
2.2
V
ns
ISYNC
SYNC Leakage
VSYNC = 0V to 5.5V
–1
0.01
1
µA
ILIM
Current Limit
PWM mode, VIN = 12V
3.8
4.7
5.7
A
RON
ISW
April 1999
Switch On-Resistance
Output Switch Leakage
skip mode
600
high-side switch, VIN = 12V
90
250
mΩ
low-side switch, VIN = 12V
110
250
mΩ
1
10
µA
VSW = 16.5V
3
mA
MIC2177
MIC2177
Symbol
Micrel
Parameter
Condition
Enable Threshold
IEN
Enable Leakage
VEN = 0V to 5.5V
AUTO Threshold
Min
Typ
Max
Units
0.8
1.6
2.2
V
–1
0.01
1
µA
0.8
1.6
7
11
V
15
µA
AUTO Source Current
VFB = 1.5V, VAUTO < 0.8V
Minimum Switch Current
for PWM Operation
VIN – VOUT = 0V
220
mA
VIN – VOUT = 3V
420
mA
General Note: Devices are ESD sensitive. Handling precautions recommended.
MIC2177
4
April 1999
MIC2177
Micrel
Typical Characteristics
180
175
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
REFERENCE VOLTAGE (V)
1.242
1.240
1.238
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Reference Voltage
vs. Temperature
5.030
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)
18.5
18.0
17.5
17.0
16.5
4.7
4.6
4.5
4.4
4.3
4.2
4
2
0
April 1999
150
100
50
2
4
4
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
3.280
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Feedback Input Bias Current
vs. Temperature
120
60
40
20
Low-Side Switch
On-Resistance
125°C
85°C
25°C
0°C
300
250
200
150
100
50
0
2
100
95
VIN = 5V
85
8V
80
12V
75
70
60
10
80
350
95
90
100
0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
SKIP
PWM
65
2
3.285
3.3V Output
Efficiency
EFFICIENCY (%)
SUPPLY CURRENT (mA)
6
3.290
100
OUTPUT
SWITCHING
8
125°C
85°C
25°C
0°C
200
0
PWM-Mode
Supply Current
10
3.295
High-Side Switch
On-Resistance
250
4.1
4.0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
12
3.300
16.0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
ON-RESISTANCE (mΩ)
4.9
4.8
3.305
Error-Amplifier Gain
vs. Temperature
19.0
Current Limit
vs. Temperature
5.0
CURRENT LIMIT (A)
1.244
3.310
BIAS CURRENT (nA)
185
1.246
MIC2177-3.3
3.315
ON-RESISTANCE (mΩ)
190
1.248
Reference Voltage
vs. Temperature
3.320
100
1000 2500
OUTPUT CURRENT (mA)
5
EFFICIENCY (%)
195
MIC2177 [adj.]
1.250
AMPLIFIER VOLTAGE GAIN
FREQUENCY (kHz)
200
Reference Voltage
vs. Temperature
1.252
REFERENCE VOLTAGE (V)
205
REFERENCE VOLTAGE (V)
Oscillator Frequency
vs. Temperature
4
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
5V Output
Efficiency
VIN = 6V
90
8V
85
12V
80
75
70
10
SKIP
PWM
100
1000 2500
OUTPUT CURRENT (mA)
MIC2177
MIC2177
Micrel
Block Diagram
VIN
4.5V to 16.5V
CIN
VIN
UVLO,
Thermal
Shutdown
1
2
9
100mΩ
P-channel
20
R1
R2
1
L1
3
VOUT
8
D
EN
Enable
Shutdown
1.245
SW
ISENSE
Amp.
Output
Control
Logic
VOUT
3.3V
Regulator
100mΩ
N-channel
COUT
PGND
4
BIAS
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
MIC2177
6
14
15
16
17
April 1999
MIC2177
Micrel
Frequency Synchronization
The MIC2177 operates at a preset switching frequency of
200kHz. 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 220kHz–300kHz, and can operate with a minimum
pulse-width of 500ns. If synchronization is not required,
connect SYNC to ground.
Functional Description
Micrel’s 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 200mA, 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.
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).
PWM-Mode Operation
During PWM operation, the MIC2177 uses current-mode
control which provides superior line regulation and makes the
control loop easier to compensate. The PWM switching
frequency is set internally to 200kHz and can be synchronized to an external clock frequency up to 300kHz. Other
features include a low-current shutdown mode, current limit,
undervoltage lockout, and thermal shutdown. See the following sections for details.
Switch Output
The switch output (SW) is a half H-bridge consisting of a highside P-channel and low-side N-channel power MOSFET.
These MOSFETs have a typical on-resistance of 100mΩ
when the MIC2177 operates from a 12V supply. Antishootthrough circuitry prevents the P-channel and N-channel from
turning on at the same time.
Refer to “PWM-Mode Functional 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 Nchannel 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-tosource 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 Pchannel 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.
Current Limit
The MIC2177 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 4.7A.
When it does, the P-channel is turned off until the next
switching period begins.
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.
Thermal Shutdown
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
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.
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 5µA.
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 0.01µF capacitor. Do not
connect any external load to the BIAS pin. It is not designed
to provide an external supply voltage.
April 1999
7
MIC2177
MIC2177
Micrel
Changing from PWM to Skip Mode
Refer to “Block Diagram” for circuits described in the following
sections.
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.
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 420mA. This is done by the
minimum current comparator which detects if the output PChannel current equals 420mA 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) = 420mA is
given by the following equation:
To prevent shoot through current, the output stage employs
break-before-make circuitry that provides approximately 50ns
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.
Skip-Mode Operation
Refer to “Skip-Mode Functional Diagram” which is a simplified block diagram of the MIC2177 operating in skip mode
and its associated waveforms.
IMIN =
Where:
∆IL1 = inductor ripple current
This equation shows IMIN varies as a function of ∆IL . Therefore, the user must select an inductor value that results in
IMIN = 200mA when IL(peak) = 420mA. The formulas for calculating the correct inductor value are given in the “Applications Information” section. Note 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 130mA to 250mA.
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.
Changing from Skip to PWM Mode
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 600mA. 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, which begins
another switching cycle.
The MIC2177 will automatically change from skip to PWM
mode when ILOAD exceeds 300mA. During skip-mode operation, it can supply up to 300mA, 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 100mA to 300mA.
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 2.2nF capacitor connected to
AUTO is charged by a 10µA 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.
The skip-mode comparator regulates VOUT by controlling
when the MIC2177 skips cycles. It compares VFB to VREF and
has 10mV of hysteresis to prevent oscillations in the control
loop. When VFB is less than VREF – 5mV, the comparator
output is logic 1, allowing the P-channel to turn on. Conversely, when VFB is greater than VREF + 5mV, 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 300mA of load
current (ILOAD) when operating in skip mode.
MIC2177
420mA − ∆IL1
2
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.
8
April 1999
MIC2177
Micrel
PWM-Mode Functional Diagram
VIN
4.5V to 16.5V
CIN
VIN
1
2
9
100mΩ
P-channel
VOUT
1.245
R1
R2
1
SW
ISENSE
Amp.
L1
3
VOUT
8
IL1
D
100mΩ
N-channel
COUT
PGND
4
5
6
7
Corrective
Ramp
Stop
SYNC
18
200kHz
Oscillator
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
IL1
ILOAD
∆IL1
Error Amp.
Output
ISENSE
April 1999
9
MIC2177
MIC2177
Micrel
Skip-Mode Functional Diagram
VIN
4.5V to 16.5V
CIN
VIN
1
2
9
Output Control Logic
S
Q
100mΩ
P-channel
R
One
Shot
ISENSE
Amp.
VOUT
1.245
R1
R2
1
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 Path
SGND
VSW
14
15
16
17
VIN
VOUT
0
One-Shot
Pulse
ILIM
IL1
0
VREF + 5mV
VFB
VREF – 5mV
MIC2177
10
April 1999
MIC2177
Micrel
To maximize efficiency, the inductor’s resistance must be
less than the output switch on-resistance (preferably 50mΩ
or less).
Output Capacitor Selection
Application Information
Feedback Resistor Selection (Adjustable Version)
The output voltage is configured by connecting an external
resistive divider to the FB pin as shown in “MIC2177 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 20kΩ. However, to
prevent feedback error due to input bias current at the FB pin,
R2 should not be greater than 100kΩ. After selecting R2,
calculate R1 using the following formula:
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
“Inductor Selection,” the maximum value for ∆IL is 600mA.
Therefore, the maximum value of ESR is:
ESRMAX =
 V


R1 = R2  OUT  − 1
 1.245V 

Where:
VRIPPLE < 1% of VOUT
Typically, capacitors in the range of 100µF to 220µF 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.
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-ofthumb, 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:
IRMS(max) =
Output Diode Selection
In PWM operation, inductor current flows through the output
diode approximately 50ns 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 ultrafast-recovery diode
(tR < 100ns) to minimize power dissipation from the diode’s
reverse-recovery charge.
Compensation
ILOAD(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 inches, 5mm). Also place a 0.1µF ceramic bypass
capacitor as close as possible to VIN.
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 600mA, and is calculated using the following formula:
Compensation is provided by connecting a series RC load to
the COMP pin. This creates a pole-zero pair in the regulator
control loop, allowing the regulator to remain stable with
enough low frequency loop-gain for good load and line
regulation. At higher frequencies, pole-zero reduces loopgain to a level referred to as the mid-band gain. The mid-band
gain is low enough so that the loop gain crosses 0dB with
sufficient phase margin. Typical values for the RC load are
4.7nF – 10nF for the capacitor and 5kΩ – 20kΩ for the
resistor.
Printed Circuit Board Layout

VOUT 
LMIN = VOUT 1 –
 × 8.3µH/V
VIN(max) 

Where:
VIN(max) = 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 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:
A well designed PC board will prevent switching noise and
ground bounce from interfering with the operation of the
MIC2177. A good design takes into consideration component
placement and routing of power traces.
The first thing to consider is the locations of the input
capacitor, inductor, output diode, and output capacitor. The
input capacitor must be placed very close to the VIN pin, the
inductor and output diode very close to the SW pin, and the
output capacitor near the inductor. These components pass
large high-frequency current pulses, so they must use short,
wide power traces. In addition, their ground pins and PGND
are connected to a ground plane that is nearest the power
supply ground bus.
IL(peak) = ILOAD(max) + 300mA
April 1999
600mA
VRIPPLE
11
MIC2177
MIC2177
Micrel
The feedback resistors, RC compensation network, and
BIAS pin bypass capacitor should be located near their
respective pins. To prevent ground bounce, their ground
traces and SGND should not be in the path of switching
currents returning to the power supply ground bus. SGND
and PGND should be tied together by a ground plane that
extends under the MIC2177.
VIN
4.5V to 16.5V
C1
22µF
35V
U1
1,2,9
VIN
20
OUT
EN
SW
18
SYNC MIC2177
11
AUTO
PGND
FB
COMP SGND BIAS
13
C5
0.01
µF
19
14–17
R4
10k
C4
6.8nF
10
L1, 50µH
3,8
4–7
VOUT
3.3V/1A
D1
MBRS130L
C2
100µF
10V
12
U1
C1
C2
C3
C3
C4
0.01µF D1
L1
L1
L1
Micrel
AVX
AVX
Z5UorX7R
X7RorNP0
Motorola
Coiltronics
Coilcraft
Bi
MIC2177-3.3BWM
TPSE226M035R0300, ESR = 0.3Ω
TPSD107M010R0100, ESR = 0.1Ω
Ceramic Dielectric Material
Ceramic Dielectric Material
MBRS130LT3
CTX50-4P, DCR = 0.097Ω
DO3316P-473, DCR = 0.12Ω
HM77-11003, DCR = 0.073Ω
Figure 1. MIC2177 4.5V–16.5V to 3.3/1A Regulator
Suggested Manufacturers List
Inductors
Capacitors
Diodes
Transistors
Coilcraft
1102 Silver Lake Rd.
Cary, IL 60013
tel: (708) 639-2361
fax: (708) 639-1469
AVX Corp.
801 17th Ave. South
Myrtle Beach, SC 29577
tel: (803) 448-9411
fax: (803) 448-1943
General Instruments (GI)
10 Melville Park Rd.
Melville, NY 11747
tel: (516) 847-3222
fax: (516) 847-3150
Siliconix
2201 Laurelwood Rd.
Santa Clara, CA 96056
tel: (800) 554-5565
Coiltronics
6000 Park of Commerce Blvd.
Boca Raton, FL 33487
tel: (407) 241-7876
fax: (407) 241-9339
Sanyo Video Components Corp.
2001 Sanyo Ave.
San Diego, CA 92173
tel: (619) 661-6835
fax: (619) 661-1055
International Rectifier Corp.
233 Kansas St.
El Segundo, CA 90245
tel: (310) 322-3331
fax: (310) 322-3332
Bi Technologies
4200 Bonita Place
Fullerton, CA
tel: (714) 447-2345
fax: (714) 447-2500
Sprague Electric
Lower Main St.
60005 Sanford, ME 04073
tel: (207) 324-4140
Motorola Inc.
MS 56-126
3102 North 56th St.
Phoenix, AZ 85018
tel: (602) 244-3576
fax: (602) 244-4015
MIC2177
12
April 1999
MIC2177
Micrel
Package Information
PIN 1
DIMENSIONS:
INCHES (MM)
0.301 (7.645)
0.297 (7.544)
0.027 (0.686)
0.031 (0.787)
0.094 (2.388)
0.090 (2.286)
0.050 (1.270)
TYP
0.016 (0.046)
TYP
0.509 (12.929)
0.505 (12.827)
0.103 (2.616)
0.099 (2.515)
7°
TYP
0.015
R
(0.381)
0.015
(0.381)
SEATING MIN
PLANE
0.297 (7.544)
0.293 (7.442)
0.330 (8.382)
0.326 (8.280)
0.022 (0.559)
0.018 (0.457)
5°
TYP
10° TYP
0.032 (0.813) TYP
0.408 (10.363)
0.404 (10.262)
20-Lead Wide SOP (WM)
April 1999
13
MIC2177
MIC2177
MIC2177
Micrel
14
April 1999
MIC2177
April 1999
Micrel
15
MIC2177
MIC2177
Micrel
MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131
TEL
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
USA
http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or
other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.
© 1999 Micrel Incorporated
MIC2177
16
April 1999
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