MICREL MIC2179

Micrel
MIC2179
MIC2179
1.5A Synchronous Buck Regulator
General Description
Features
The Micrel MIC2179 is a 200kHz synchronous buck (stepdown) switching regulator designed for high-efficiency, battery-powered applications.
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
5µA.
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 200kHz and can be
synchronized to an external clock signal of up to 300kHz.
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 and is packaged in a 20-lead SSOP package
with an operating temperature range of –40°C to +85°C.
• 4.5V to 16.5V input voltage range
• Dual-mode operation for high efficiency (up to 96%)
PWM mode for > 150mA load current
Skip mode for <150mA load current
• 150mΩ internal power MOSFETs at 12V input
• 200kHz preset switching frequency
• Low quiescent current
1.0mA in PWM mode
600µA in skip mode
< 5µA 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
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 16.5V
C1
10µF
20V
Output Good
Output Low
Skip Mode
PWM Mode
R1
20k
U1
15
6
5
13
16,17
VIN
EN
SW
PWRGD
PWM
MIC
2179-3.3 PGND
SYNC
FB
COMP SGND
8
9–12
R2
10k
3,4
1,2,
19,20
D1
MBRM120
VOUT
3.3V/600mA
C2
100µF
6.3V
7
BIAS
14
C3
0.01µF
C4
6.8nF
L1
22µH
Pins 4 and 18 are not connected.
Pins 3 and 4 can be connected
together for a low-impedance
connection.
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
June 1998
1
MIC2179
Micrel
MIC2179
Ordering Information
Part Number
Voltage
Temperature Range
Package
MIC2179BSM
Adjustable
–40°C to +85°C
20-lead SSOP
MIC2179-3.3BSM
3.3V
–40°C to +85°C
20-lead SSOP
MIC2179-5.0BSM
5.0V
–40°C to +85°C
20-lead SSOP
Pin Configuration
PGND 1
20 PGND
PGND 2
19 PGND
SW 3
18 NC
NC 4
17 VIN
PWM 5
16 VIN
15 EN
PWRGD 6
FB 7
14 BIAS
COMP 8
13 SYNC
SGND 9
12 SGND
SGND 10
11 SGND
20-Lead Wide SSOP
Pin Description
Pin Number
Pin Name
1, 2, 19, 20
PGND
3
SW
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
8
COMP
Compensation: Output of internal error amplifier. Connect capacitor or
series RC network to compensate the regulator control loop.
9–12
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 0.01µF 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.
4, 18
NC
not internally connected.
MIC2179
Pin Function
Power Ground: Connect all pins to central ground point.
Switch (Output): Internal power MOSFET output switches.
Feedback (Input): Connect to output voltage divider resistors.
2
June 1998
Micrel
MIC2179
Absolute Maximum Ratings
Operating Ratings
Supply Voltage [100ms 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
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
600
750
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
MIC2179 [adj.]: VOUT = 3.3V, ILOAD = 0
1.22
1.245
1.27
V
VOUT
Output Voltage
MIC2179 [adj.]: VOUT = 3.3V,
5V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 1A
3.20
3.14
3.3
3.40
3.46
V
V
MIC2179-5.0: ILOAD = 0
4.85
5.0
5.15
V
MIC2179-5.0:
6V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 1A
4.85
4.75
5.0
5.15
5.25
V
MIC2179-3.3: ILOAD = 0
3.20
3.3
3.40
V
MIC2179-3.3:
5V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 1A
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
3.90
4.15
MIC2179 [adj.]
60
150
nA
MIC2179-5.0, MIC2179-3.3
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
3.4
4.3
5.5
A
RON
ISW
June 1998
Switch On-Resistance
Output Switch Leakage
skip mode
600
high-side switch, VIN = 12V
160
350
mΩ
low-side switch, VIN = 12V
140
350
mΩ
1
10
µA
VSW = 16.5V
3
mA
MIC2179
Micrel
MIC2179
Symbol
Parameter
Condition
Enable Threshold
IEN
Enable Leakage
VEN = 0V to 5.5V
PWM Threshold
IPWM
Min
Typ
Max
Units
0.8
1.6
2.2
V
–1
0.01
1
µA
0.6
1.1
1.4
V
–1
0.01
1
µA
PWM Leakage
VPWM = 0V to 5.5V
PWRGD Threshold
MIC2179 [adj.]: measured at FB pin
1.09
1.13
1.17
V
MIC2179-5.0: measured at FB pin
4.33
4.54
4.75
V
MIC2179-3.3: measured at FB pin
2.87
3.00
3.13
V
PWRGD Output Low
ISINK = 1.0mA
0.25
0.4
V
PWRGD Off Leakage
VPWRGD = 5.5V
0.01
1
µA
General Note: Devices are ESD sensitive. Handling precautions recommended.
MIC2179
4
June 1998
Micrel
MIC2179
Typical Characteristics
185
180
175
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
REFERENCE VOLTAGE (V)
1.244
1.242
1.240
1.238
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Reference Voltage
vs. Temperature
5.030
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)
18.5
18.0
17.5
17.0
16.5
4.9
4.7
4.5
4.3
4.1
3.9
250
200
150
100
4
2
0
2
June 1998
4
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
3.290
3.285
3.280
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Feedback Input Bias Current
vs. Temperature
120
4
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
100
80
60
40
20
0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
Low-Side Switch
On-Resistance
400
125°C
85°C
25°C
0°C
350
300
250
200
150
100
50
0
2
4
6 8 10 12 14 16 18
INPUT VOLTAGE (V)
Skip- and PWM-Mode
Efficiency
90
5.4V
PWM
85
8.4V
Skip
80
75
8.4V
PWM
70
65
2
3.295
95
EFFICIENCY (%)
SUPPLY CURRENT (mA)
6
50
0
OUTPUT
SWITCHING
8
125°C
85°C
25°C
0°C
300
PWM-Mode
Supply Current
10
3.300
High-Side Switch
On-Resistance
350
3.7
3.5
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
12
3.305
16.0
-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)
ON-RESISTANCE (mΩ)
5.3
5.1
3.310
Error-Amplifier Gain
vs. Temperature
19.0
Current Limit
vs. Temperature
5.5
CURRENT LIMIT (A)
1.246
MIC2179-3.3
3.315
BIAS CURRENT (nA)
190
1.248
Reference Voltage
vs. Temperature
3.320
ON-RESISTANCE (mΩ)
195
MIC2179 [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
60
10
5.4V
Skip
100
600
OUTPUT CURRENT (mA)
5
MIC2179
Micrel
MIC2179
Block Diagram
VIN
4.5V to 16.5V
100µF
VIN
UVLO,
Thermal
Shutdown
EN
Enable
Shutdown
15
16
17
VOUT
110mΩ
P-channel
ISENSE
Amp.
Output
Control
Logic
3.3V
Regulator
1.245
L
SW
D
110mΩ
N-channel
14
0.01µF
COUT
PGND
PWM
5
2
ILIMIT
Comp.
SYNC
13
*
19
* Connect
SGND to PGND
20
PWM/
Skip-Mode
Select
ILIMIT
Thresh.
Voltage
Bold lines indicate
high current traces
Corrective
Ramp
200kHz
Oscillator
R1
Skip-Mode
Comp.
Reset
Pulse
FB
R
7
Q
S
VIN
Power Good
Comp.
PWM
Comp.
RC
VOUT
1
internal
supply Voltage
Stop
1
3
BIAS
Skip Mode
PWM Mode
R1
R2
R2
20k
PWRGD
Output Good
6
COMP
8
CC
1.13V
VREF 1.245V
MIC2179 [Adjustable]
SGND
MIC2179
6
9
10
11
12
June 1998
Micrel
MIC2179
connect an external load to the BIAS pin. It is not designed to
provide an external supply voltage.
Frequency Synchronization
The MIC2179 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 to 300kHz, and can operate with a minimum
pulse width of 500ns. If synchronization is not required,
connect SYNC to ground.
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 to low. PWRGD is an open-drain output
that can sink 1mA from a pull-up resistor connected to VIN.
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
Refer to “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 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.
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
Functional Description
Micrel’s 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. Its 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 150mA, 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 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 more detail.
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 150mΩ
when the MIC2179 operates from a 12V supply. Antishootthrough circuitry prevents the P-channel and N-channel from
turning on at the same time.
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.
Undervoltage Lockout
Undervoltage lockout (UVLO) turns off the output when the
input voltage (VIN) is to 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 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.
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 5µA.
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 0.01µF capacitor. Do not
June 1998
7
MIC2179
Micrel
MIC2179
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 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 MIC2179 to always supply up to 300mA of load
current when operating in skip mode.
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 150mA, 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.
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 current of approximately 300mA, 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
200mA.
(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 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 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 400mA. When IL1 reaches this value, the current limit
comparator sets the RS latch output to logic 0, turning off the
MIC2179
8
June 1998
Micrel
MIC2179
PWM-Mode Functional Diagram
VIN
4.5V to 16.5V
CIN
VIN
16
17
VOUT
110mΩ
P-channel
ISENSE
Amp.
1.245
R1
R2
1
L1
SW
VOUT
3
IL1
D
110mΩ
N-channel
COUT
PGND
1
2
19
20
Corrective
Ramp
Stop
SYNC
13
200kHz
Oscillator
R1
Reset
Pulse
FB
7
R2
R
Q
S
PWM
Comp.
Error
Amp.
COMP
CC
RC
8
VREF 1.245V
MIC2179 [Adjustable] PWM-Mode Signal Path
SGND
9
10
11
12
VSW
Reset
Pulse
IL1
ILOAD
∆IL1
Error Amp.
Output
ISENSE
June 1998
9
MIC2179
Micrel
MIC2179
Skip-Mode Functional Diagram
VIN
4.5V to 16.5V
CIN
VIN
16
17
Output Control Logic
S
VOUT
Q
1.245
110mΩ
P-channel
R
One
Shot
ISENSE
Amp.
R1
R2
1
L1
SW
VOUT
3
IL1
D
COUT
PGND
1
2
ILIMIT
Comp.
19
20
ILIMIT
Thresh.
Voltage
R1
Skip-Mode
Comp.
FB
7
R2
VREF 1.245V
MIC2179 [Adjustable] Skip-Mode Signal Path
SGND
VSW
9
10
11
12
VIN
VOUT
0
One-Shot
Pulse
ILIM
IL1
0
VREF + 5mV
VFB
VREF – 5mV
MIC2179
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June 1998
Micrel
MIC2179
To maximize efficiency, the inductor’s resistance must be
less than the output switch on-resistance (preferably,
50mΩ or less).
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. 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 ∆IL(max):
Application Information
Feedback Resistor Selection (Adjustable Version)
The output voltage is programmed by connecting an external
resistive divider to the FB pin as shown in “MIC2179 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 with the following formula:
 V
 
R1 = R2   OUT  – 1
  1.245V  
ESRMAX =
Typically, capacitors in the range of 100 to 220µF 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 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.
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 the other turns on. In skip mode, the
inductor current flows through the diode during the entire Pchannel 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
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, the pole-zero reduces
loop-gain to a level referred to as the mid-band gain. The midband gain is low enough so that the loop gain crosses 0db
with sufficient phase margin. Typical values for the RC load
are 4.7nF to 10nF for the capacitor and 5kΩ to 20kΩ for the
resistor.
Printed Circuit Board Layout
A well designed PC board will prevent switching noise and
ground bounce from interfering with the operation of the
MIC2179. 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.
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) =
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 0.2in, 5mm). Also, place a 0.1µF ceramic bypass
capacitor as close as possible to VIN.
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:
LMIN = VOUT × 3.0µH/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:
IL(peak) = ILOAD(max) +
∆IL(max)
2
Where:

VOUT 
5µs
∆IL(max) = VOUT 1 −
 ×
V
L

IN(max) 
June 1998
VRIPPLE
∆IL(max)
11
MIC2179
Micrel
MIC2179
The feedback resistors, RC compensation network, and
BIAS pin bypass capacitor should be located close to 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 MIC2179.
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
MIC2179
12
June 1998
Micrel
MIC2179
Package Information
5.40 (0.213)
5.20 (0.205)
7.90 (0.311)
7.65 (0.301)
DIMENSIONS:
MM (INCH)
0.875 (0.034)
REF
7.33 (0.289)
7.07 (0.278)
2.00 (0.079)
1.73 (0.068)
0.38 (0.015)
0.25 (0.010)
0.21 (0.008)
0.05 (0.002)
0.65 (0.0260)
COPLANARITY:
BSC
0.10 (0.004) MAX
10°
4°
0°
–8°
0.22 (0.009)
0.13 (0.005)
1.25 (0.049) REF
0.95 (0.037)
0.55 (0.022)
20-lead SSOP (SM)
June 1998
13
MIC2179
Micrel
MIC2179
MIC2179
14
June 1998
Micrel
MIC2179
June 1998
15
MIC2179
Micrel
MIC2179
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.
© 1998 Micrel Incorporated
MIC2179
16
June 1998