MICREL MIC2177

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.
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.
The MIC2177 is packaged in a 20-pin 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.
Data sheets and support documentation can be found on
Micrel’s web site at: www.micrel.com.
• 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
Applications
• High-efficiency, battery-powered supplies
• Buck (step-down) dc-to-dc converters
• Cellular telephones
• Laptop computers
• Hand-held instruments
• Battery Charger
_________________________________________________________________________________________________________
Typical Application
VIN
5.4V to 18V
100
U1
1,2,9
95
VIN
ENABLE
20
SHUTDOWN
18
11
2.2
nF
OUT
EN
SW
MIC
SYNC 2177-5.0
PGND
AUTO
FB
COMP SGND BIAS
R1
10k
CC
6.8nF
13
14–17
10
3,8
4–7
L1, 50µH
D1
MBRS130L
12
19
C3
0.01µF
VOUT
5V/1A
C2
100µF
10V
EFFICIENCY (%)
C1
22µF
35V
5V Output
Efficiency
90
85
80
75
R1
10k
VIN = 6V
70
10
SKIP
PWM
100
1000 2500
OUTPUT CURRENT (mA)
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
April 2008
M9999-042108
Micrel, Inc.
MIC2177
Ordering Information
Part Number
Output
Voltage
Switching
Frequency
Temperature Range
Package
Lead Finish
MIC2177-3.3BWM
3.3V
200kHz
–40°C to +85°C
20-Pin Wide SOIC
Standard
MIC2177-5.0BWM
5.0V
200kHz
–40°C to +85°C
20-Pin Wide SOIC
Standard
MIC2177BWM
Adj.
200kHz
–40°C to +85°C
20-Pin Wide SOIC
Standard
MIC2177-3.3YWM
3.3V
200kHz
–40°C to +85°C
20-Pin Wide SOIC
Pb-Free
Pin Configuration
VIN 1
20 E N
VIN 2
19 B I A S
SW 3
18 S Y NC
P GND 4
17 S G N D
P GND 5
16 S G N D
P GND 6
15 S G N D
P GND 7
14 S G N D
SW 8
13 C O M P
VIN 9
12 F B
11 AUTO
OUT 10
20-Pin Wide SOIC (WM)
April 2008
2
M9999-042108
Micrel, Inc.
MIC2177
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
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 0.01µF bypass
capacitor and 10kΩ to SGND. Do not apply any external load.
20
EN
April 2008
Pin Function
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 VOUT.
Automatic Mode: Connect 2.2nF 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.
3
M9999-042108
Micrel, Inc.
MIC2177
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 (TJ) ........................ –40°C to +125°C
Electrical Characteristics
VIN = 7.0V; TA = 25°C, bold values indicate –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
VFB
VOUT
VTH
VIN = 16.5V
3.10
3.30
3.40
V
Feedback Voltage
MIC2177 [adj.]: VOUT = 3.3V, ILOAD = 0
1.22
1.245
1.27
V
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
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
Undervoltage Lockout
VTL
IFB
AVOL
upper threshold
lower threshold
Feedback Bias Current
3.9
4.15
60
150
nA
MIC2177-5.0, MIC2177-3.3
20
40
µA
20
V
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
MIC2177 [adj.]
0.05
source and sink
fO
Oscillator Frequency
DMAX
Maximum Duty Cycle
VFB = 1.0V
tON min
Minimum On-Time
VFB = 1.5V
V
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
April 2008
Switch On-Resistance
skip mode
600
high-side switch, VIN = 12V
90
250
mΩ
low-side switch, VIN = 12V
110
250
mΩ
4
mA
M9999-042108
Micrel, Inc.
MIC2177
Symbol
Parameter
Condition
ISW
Output Switch Leakage
VSW = 16.5V
Min
Enable Threshold
IEN
Enable Leakage
VEN = 0V to 5.5V
AUTO Threshold
Typ
Max
Units
1
10
µA
0.8
1.6
2.2
V
1
µA
–1
0.01
0.8
1.6
7
11
V
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
15
µA
General Note: Devices are ESD sensitive. Handling precautions recommended.
April 2008
5
M9999-042108
Micrel, Inc.
MIC2177
Typical Characteristics
190
185
180
175
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
1.248
1.246
1.244
1.242
1.240
1.238
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Reference Voltage
vs. Temperature
5.030
MIC21775.0
5.020
5.010
5.000
4.990
4.980
3.315
18.0
17.5
17.0
16.5
3.305
3.300
3.295
3.290
3.285
3.280
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Feedback Input Bias Current
vs. Temperature
120
16.0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Current Limit
vs. Temperature
4.9
4.8
200
4.7
4.6
150
100
4.2
4.1
50
0
PWM-Mode
Supply Current
12
125°C
85°C
25°C
0°C
40
20
Low-Side Switch
On-Resistance
125°C
85°C
25°C
0°C
300
250
200
150
10
8
6
4
2
50
2
100
OUTPUT
SWITCHING
4
3.3V Output
Efficiency
90
April 2008
4
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
85
8V
80
12V
75
70
60
10
2
100
95
VIN = 5V
SKIP
PWM
65
2
0
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
95
EFFICIENCY (%)
SUPPLY CURRENT (mA)
60
100
4.0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
0
80
350
4.5
4.4
4.3
100
0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
High-Side Switch
On-Resistance
250
MIC2177-3.3
3.310
100
1000 2500
OUTPUT CURRENT (mA)
6
EFFICIENCY (%)
CURRENT LIMIT (A)
18.5
Reference Voltage
vs. Temperature
3.320
Error-Amplifier Gain
vs. Temperature
19.0
4.970
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
5.0
MIC2177 [adj.]
BIAS CURRENT (nA)
195
1.250
AMPLIFIER VOLTAGE GAIN
FREQUENCY (kHz)
200
REFERENCE VOLTAGE (V)
1.252
REFERENCE VOLTAGE (V)
205
Reference Voltage
vs. Temperature
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
80
75
70
10
12V
SKIP
PWM
100
1000 2500
OUTPUT CURRENT (mA)
M9999-042108
Micrel, Inc.
MIC2177
Functional 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
0.01µF
5
ILIMIT
Comp.
19
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
2.2nF
R2
11
40mV
Skip-Mode
Comp.
RESET PULSE
Q
R
S
PWM
Comp.
Error
Amp.
COMP
RC
CC
VREF
1.245V
13
MIC2177 [Adjustable]
SGND
April 2008
7
14
15
16
17
M9999-042108
Micrel, Inc.
MIC2177
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.
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.
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.
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.
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.
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 100mΩ when the MIC2177 operates from a 12V
supply. Anti-shoot-through circuitry prevents the Pchannel and N-channel from turning on at the same
time.
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).
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.
PWM-Mode Operation
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 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 Pchannel and turns on the N-channel. Energy is then
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
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.
April 2008
8
M9999-042108
Micrel, Inc.
MIC2177
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 I L1 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 de-creases 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 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.
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 MIC2177 uses current-mode control to adjust the
duty cycle and regulate the output voltage. Currentmode 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 Nchannel 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 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.
Changing from PWM to Skip Mode
Refer to “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 420mA. This is
done by the minimum current comparator which detects
if the output P-Channel 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:
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.
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 Nchannel is kept off to optimize efficiency by reducing
gate charge dissipation. VOUT is regulated by skipping
April 2008
IMIN =
420mA − ∆IL1
2
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
9
M9999-042108
Micrel, Inc.
MIC2177
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
thresh-old 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.
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.
Changing from Skip to PWM Mode
The MIC2177 will automatically change from skip to
PWM mode when ILOAD exceeds 300mA. During skipmode 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.
April 2008
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.
10
M9999-042108
Micrel, Inc.
MIC2177
PWM-Mode Functional Diagram
VIN
4.5V to 16.5V
CIN
VIN 1
2
9
R1
VOUT = 1.245 (
+ 1)
R2
100m
P-channel
SW
IS E N S E
Amp.
L1
3
8
IL1
D
100m
N-channel
VO U T
COU T
P GND
4
5
6
7
Stop
S Y NC
18
Corrective
Ramp
200kHz
Oscillator
R1
Reset
Pulse
FB
12
R2
Q
R
S
PWM
Comp.
Error
Amp.
COMP
CC
RC
13
VR E F1.245V
MIC2177 [Adjustable] PWM-Mode Signal Path
S G N D 14
15
16
17
VS W
Reset
Pulse
IL1
I LOAD
IL1
Error Amp.
Output
ISE NS E
April 2008
11
M9999-042108
Micrel, Inc.
MIC2177
Skip-Mode Functional Diagram
VIN
4.5V to 16.5V
CIN
VIN 1
2
9
Output Control Logic
S
R
Q
R1
VOUT = 1.245 (
+ 1)
R2
100m
P-channel
One
Shot
IS E N S E
Amp.
SW
L1
3
8
VO U T
IL1
D
CO U T
P GND
4
5
ILIMIT
Comp.
6
7
ILIMIT
Thresh.
Voltage
R1
Skip-Mode
Comp.
FB
12
R2
VR E F1.245V
MIC2177 [Adjustable] Skip-Mode Signal Pat
S G N D 14
VS W
15
16
17
VIN
VO U T
0
One-Shot
Pulse
I LIM
IL1
0
VR E F + 5mV
VF B
VR E F – 5mV
April 2008
12
M9999-042108
Micrel, Inc.
MIC2177
increase significantly. Both of these effects will degrade
efficiency. The formula for peak inducto rcurrent is:
IL(peak) = ILOAD(max) + 300mA
To maximize efficiency, the inductor’s resistance must
be less than the output switch on-resistance (preferably
50mΩor less).
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:
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 “Inductor Selection,” the maximum value
for ∆IL is 600mA.
Therefore, the maximum value of ESR is:
⎡⎛ V
⎞ ⎤
R1 = R2⎢⎜⎜ OUT ⎟⎟ − 1⎥
⎣⎝ 1.245V ⎠ ⎦
ESR MAX =
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:
IRMS(max) =
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.
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.
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 ultra fast-recovery diode (tR<100ns) to
minimize power dissipation from the diode’s reverserecovery charge.
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:
⎛
VOUT
L MIN = VOUT ⎜1 −
⎜
VIN(max)
⎝
Compensation
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 loop-gain 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.
⎞
⎟ × 8.3µ.3µ
⎟
⎠
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
April 2008
600mA
VRIPPLE
Printed Circuit Board Layout
A well designed PC board will prevent switching noise
and ground bounce from interfering with the operation of
13
M9999-042108
Micrel, Inc.
MIC2177
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.
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
18
11
EN
SW
SYNC MIC2177
PGND
AUTO
COMP SGND
C5
0.01
µF
OUT
R4
10k
13
C4
6.8nF
FB
BIAS
14–17
C3
0.01µF
10
L1, 50µH
3,8
D1
MBRS130L
4–7
VOUT
3.3V/1A
C2
100µF
10V
12
U1
C1
C2
C3
C4
D1
L1
L1
L1
19
R4
10k
Micrel
MIC2177-3.3BWM
AVX
AVX
Z5UorX7R Ceramic Dielectric Material
X7RorNP0 Ceramic Dielectric Material
Motorola MBRS130LT3
Coiltronics CTX50-4P, DCR = 0.097
Coilcraft
Bi
HM77-11003, DCR = 0.073
Bill of Materials
Inductors
Capacitors
Diodes
Transistors
Coilcraft
AVX
General Instruments (GI)
Siliconix
1102 Silver Lake Rd.
Cary, IL 60013
Tel: (708) 639-2361
Fax: (708) 639-1469
801 17 Ave.
Myrtle Beach, SC 29577
Tel: (803) 448-9411
Fax: (803) 448-1973
10 Melville Park Rd.
Melville, NY 11747
Tel: (516) 847-3222
Fax: (516) 847-3150
2201 Laurelwood Rd.
Santa Clara, CA 96056
Tel: (800) 554-5565
Coiltronics
Sanyo Video Components Corp.
International Rectifier Corp.
6000 Park of Commerce Blvd.
Boca Raton, FL 33487
Tel: (407) 241-7876
Fax: (407) 241-9339
2001 Sanyo Ave.
San Diego, CA 92173
Tel: (619) 661-6835
Fax: (619) 661-1055
233 Kansas St.
El Segundo, CA 90245
Tel: (310) 322-3331
Fax: (310) 322-3332
Bi Technologies
Sprague Electric
Motorola, Inc.
4200 Bonita Place
Fullerton, CA 92835
Tel: (714) 447-2345
Fax: (714) 447-2500
60005 Lower Main St.
Sanford, ME 04073
Tel: (207) 324-4140
MS 56-126
th
3102 North 56 St.
Phoenix, AZ 85018
Tel: (602) 244-3576
Fax: (602) 244-4015
April 2008
th
14
M9999-042108
Micrel, Inc.
MIC2177
Package Information
20-Pin Wide SOIC (WM)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its
use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant
into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully
indemnify Micrel for any damages resulting from such use or sale.
© 1999 Micrel, Incorporated.
April 2008
15
M9999-042108