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Электронный компонент: MIC2179-5.0BSM

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June 1998
1
MIC2179
MIC2179
Micrel
MIC2179
1.5A Synchronous Buck Regulator
General Description
The Micrel MIC2179 is a 200kHz synchronous buck (step-
down) switching regulator designed for high-efficiency, bat-
tery-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
5
A.
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 intelli-
gent 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 regula-
tion and makes the regulator control loop easy to compen-
sate. 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.
Typical Application
BIAS
SGND
COMP
PGND
FB
SW
VIN
C2
100F
6.3V
V
OUT
3.3V/600mA
L1
22H
C4
6.8nF
C3
0.01F
MIC
2179-3.3
PWM
Skip Mode
PWM Mode
SYNC
EN
R2
10k
V
IN
5.4V to 16.5V
C1
10F
20V
U1
15
5
13
8
912
14
7
1,2,
19,20
3,4
16,17
D1
MBRM120
PWRGD
6
R1
20k
Output Good
Output Low
Pins 4 and 18 are not connected.
Pins 3 and 4 can be connected
together for a low-impedance
connection.
Features
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
150m
internal power MOSFETs at 12V input
200kHz preset switching frequency
Low quiescent current
1.0mA in PWM mode
600
A in skip mode
< 5
A 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
Micrel, Inc. 1849 Fortune Drive San Jose, CA 95131 USA tel + 1 (408) 944-0800 fax + 1 (408) 944-0970 http://www.micrel.com
MIC2179
Micrel
MIC2179
2
June 1998
Pin Description
Pin Number
Pin Name
Pin Function
1, 2, 19, 20
PGND
Power Ground: Connect all pins to central ground point.
3
SW
Switch (Output): Internal power MOSFET output switches.
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 (V
REF
).
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.
912
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
F 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 V
IN
.
4, 18
NC
not internally connected.
Pin Configuration
2
PGND
3
SW
4
NC
5
PWRGD 6
PWM
7
FB
1
PGND
8
COMP
9
SGND
PGND
20
PGND
19
NC
18
VIN
17
VIN
16
EN
15
10
SGND
14
13
12
11 SGND
SGND
SYNC
BIAS
20-Lead Wide SSOP
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
June 1998
3
MIC2179
MIC2179
Micrel
Electrical Characteristics
V
IN
= 7.0V; T
A
= 25
C, bold indicates 40
C
T
A
85
C; unless noted.
Symbol
Parameter
Condition
Min
Typ
Max
Units
I
SS
Input Supply Current
PWM mode, output not switching,
1.0
1.5
mA
4.5V
V
IN
16.5V
skip mode, output not switching,
600
750
A
4.5V
V
IN
16.5V
V
EN
= 0V, 4.5V
V
IN
16.5V
1
25
A
V
BIAS
Bias Regulator Output Voltage
V
IN
= 16.5V
3.10
3.30
3.4
V
V
FB
Feedback Voltage
MIC2179 [adj.]: V
OUT
= 3.3V, I
LOAD
= 0
1.22
1.245
1.27
V
V
OUT
Output Voltage
MIC2179 [adj.]: V
OUT
= 3.3V,
3.20
3.3
3.40
V
5V
V
IN
16V, 10mA
I
LOAD
1A
3.14
3.46
V
MIC2179-5.0: I
LOAD
= 0
4.85
5.0
5.15
V
MIC2179-5.0:
4.85
5.0
5.15
6V
V
IN
16V, 10mA
I
LOAD
1A
4.75
5.25
V
MIC2179-3.3: I
LOAD
= 0
3.20
3.3
3.40
V
MIC2179-3.3:
3.20
3.3
3.40
V
5V
V
IN
16V, 10mA
I
LOAD
1A
3.14
3.46
V
V
TH
Undervoltage Lockout
upper threshold
4.25
4.35
V
V
TL
lower threshold
3.90
4.15
V
I
FB
Feedback Bias Current
MIC2179 [adj.]
60
150
nA
MIC2179-5.0, MIC2179-3.3
20
40
A
A
VOL
Error Amplifier Gain
0.6V
V
COMP
0.8V
15
18
20
Error Amplifier Output Swing
upper limit
0.9
1.5
V
lower limit
0.05
0.1
V
Error Amplifier Output Current
source and sink
15
25
35
A
f
O
Oscillator Frequency
160
200
240
kHz
D
MAX
Maximum Duty Cycle
V
FB
= 1.0V
100
%
t
ON min
Minimum On-Time
V
FB
= 1.5V
300
400
ns
SYNC Frequency Range
220
300
kHz
SYNC Threshold
0.8
1.6
2.2
V
SYNC Minimum Pulse Width
500
ns
I
SYNC
SYNC Leakage
V
SYNC
= 0V to 5.5V
1
0.01
1
A
I
LIM
Current Limit
PWM mode, V
IN
= 12V
3.4
4.3
5.5
A
skip mode
600
mA
R
ON
Switch On-Resistance
high-side switch, V
IN
= 12V
160
350
m
low-side switch, V
IN
= 12V
140
350
m
I
SW
Output Switch Leakage
V
SW
= 16.5V
1
10
A
Absolute Maximum Ratings
Supply Voltage [100ms transient] (V
IN
) ......................... 18V
Output Switch Voltage (V
SW
) ........................................ 18V
Output Switch Current (I
SW
) ......................................... 6.0A
Enable, PWM Control Voltage (V
EN
, V
PWM
) ................. 18V
Sync Voltage (V
SYNC
) ..................................................... 6V
Operating Ratings
Supply Voltage (V
IN
) ..................................... 4.5V to 16.5V
Junction Temperature Range (T
J
) ........... 40
C to +125
C
MIC2179
Micrel
MIC2179
4
June 1998
Symbol
Parameter
Condition
Min
Typ
Max
Units
Enable Threshold
0.8
1.6
2.2
V
I
EN
Enable Leakage
V
EN
= 0V to 5.5V
1
0.01
1
A
PWM Threshold
0.6
1.1
1.4
V
I
PWM
PWM Leakage
V
PWM
= 0V to 5.5V
1
0.01
1
A
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
I
SINK
= 1.0mA
0.25
0.4
V
PWRGD Off Leakage
V
PWRGD
= 5.5V
0.01
1
A
General Note: Devices are ESD sensitive. Handling precautions recommended.
June 1998
5
MIC2179
MIC2179
Micrel
Typical Characteristics
175
180
185
190
195
200
205
-60 -30
0
30
60
90 120 150
FREQUENCY (kHz)
TEMPERATURE (
C)
Oscillator Frequency
vs. Temperature
1.238
1.240
1.242
1.244
1.246
1.248
1.250
1.252
-60 -30
0
30
60
90 120 150
REFERENCE VOLTAGE (V)
TEMPERATURE (
C)
Reference Voltage
vs. Temperature
MIC2179 [adj.]
3.280
3.285
3.290
3.295
3.300
3.305
3.310
3.315
3.320
-60 -30
0
30
60
90 120 150
REFERENCE VOLTAGE (V)
TEMPERATURE (
C)
Reference Voltage
vs. Temperature
MIC2179-3.3
4.970
4.980
4.990
5.000
5.010
5.020
5.030
-60 -30
0
30
60
90 120 150
REFERENCE VOLTAGE (V)
TEMPERATURE (
C)
Reference Voltage
vs. Temperature
MIC2179-5.0
16.0
16.5
17.0
17.5
18.0
18.5
19.0
-60 -30
0
30
60
90 120 150
AMPLIFIER VOLTAGE GAIN
TEMPERATURE (
C)
Error-Amplifier Gain
vs. Temperature
0
20
40
60
80
100
120
-60 -30
0
30
60
90 120 150
BIAS CURRENT (nA)
TEMPERATURE (
C)
Feedback Input Bias Current
vs. Temperature
3.5
3.7
3.9
4.1
4.3
4.5
4.7
4.9
5.1
5.3
5.5
-60 -30
0
30
60
90 120 150
CURRENT LIMIT (A)
TEMPERATURE (
C)
Current Limit
vs. Temperature
0
50
100
150
200
250
300
350
2
4
6
8
10 12 14 16 18
ON-RESISTANCE (m
)
INPUT VOLTAGE (V)
High-Side Switch
On-Resistance
125
C
85
C
25
C
0
C
0
50
100
150
200
250
300
350
400
2
4
6
8
10 12 14 16 18
ON-RESISTANCE (m
)
INPUT VOLTAGE (V)
Low-Side Switch
On-Resistance
125
C
85
C
25
C
0
C
0
2
4
6
8
10
12
2
4
6
8
10 12 14 16 18
SUPPLY CURRENT (mA)
INPUT VOLTAGE (V)
PWM-Mode
Supply Current
OUTPUT
SWITCHING
60
65
70
75
80
85
90
95
10
100
600
EFFICIENCY (%)
OUTPUT CURRENT (mA)
5.4V
PWM
8.4V
PWM
8.4V
Skip
5.4V
Skip
Skip- and PWM-Mode
Efficiency
MIC2179
Micrel
MIC2179
6
June 1998
Block Diagram
SW
PGND
I
SENSE
Amp.
PWM/
Skip-Mode
Select
V
REF
1.245V
110m
N-channel
110m
P-channel
COMP
VIN
Skip-Mode
Comp.
I
LIMIT
Comp.
Output
Control
Logic
Power Good
Comp.
1.13V
R
S
Q
200kHz
Oscillator
PWM
Comp.
3.3V
Regulator
UVLO,
Thermal
Shutdown
V
OUT
L
FB
SGND
C
C
PWRGD
0.01F
EN
BIAS
V
IN
4.5V to 16.5V
100F
SYNC
C
OUT
MIC2179 [Adjustable]
internal
supply Voltage
Enable
Shutdown
Stop
15
14
13
6
8
17
16
3
D
1
2
19
20
R1
R2
7
9
10
11
12
Bold lines indicate
high current traces
I
LIMIT
Thresh.
Voltage
PWM
Skip Mode
PWM Mode
5
V
IN
Output Good
20k
*
* Connect
S
GND
to P
GND
R
C
Reset
Pulse
Corrective
Ramp
V
OUT
1.245
R1
R2
1
June 1998
7
MIC2179
MIC2179
Micrel
Functional Description
Micrel's MIC2179 is a synchronous buck regulator that oper-
ates 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 synchro-
nized 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 follow-
ing sections for more detail.
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 150m
when the MIC2179 operates from a 12V supply. Antishoot-
through 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 (V
IN
) is to low to provide sufficient gate drive for
the output MOSFETs. It prevents the output from turning on
until V
IN
exceeds 4.3V. Once operating, the output will not
shut off until V
IN
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 5
A.
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
F capacitor. Do not
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 V
OUT
is to low. PWRGD is an open-drain output
that can sink 1mA from a pull-up resistor connected to V
IN
.
Low-Dropout Operation
Output regulation is maintained in PWM or skip mode even
when the difference between V
IN
and V
OUT
decreases below
1V. As V
IN
V
OUT
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 V
IN
V
OUT
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 simpli-
fied 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 (I
L1
) increases
and energy is stored in the inductor. The current sense
amplifier (I
SENSE
Amp) measures the P-channel drain-to-
source voltage and outputs a voltage proportional to I
L1
. The
output of I
SENSE
Amp is added to a sawtooth waveform
(corrective ramp) generated by the oscillator, creating a
composite waveform labeled I
SENSE
on the timing diagram.
When I
SENSE
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 dis-
charged from the inductor and I
L1
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: V
OUT
is fed back to the error amplifier which compares
the feedback voltage (V
FB
) to an internal reference voltage
MIC2179
Micrel
MIC2179
8
June 1998
(V
REF
). When V
OUT
is lower than its nominal value, the error
amplifier output voltage increases. This voltage then inter-
sects the current sense waveform later in switching period
which increases the duty cycle and the average inductor
current . If V
OUT
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 V
IN
, V
OUT
, and
the output inductor value. While in skip mode, the N-channel
is kept off to optimize efficiency by reducing gate charge
dissipation. V
OUT
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 I
L1
to increase linearly until it reaches a current limit
of 400mA. 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 (V
SW
) then swings from
V
IN
to 0.4V below ground, and I
L1
flows through the Schottky
diode. L1 discharges its energy to the output and I
L1
de-
creases to zero. When I
L1
= 0, V
SW
swings from 0.4V to
V
OUT
, 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 V
OUT
by controlling
when the MIC2179 skips cycles. It compares V
FB
to V
REF
and
has 10mV of hysteresis to prevent oscillations in the control
loop. When V
FB
is less than V
REF
5mV, the comparator
output is logic 1, allowing the P-channel to turn on. Con-
versely, when V
FB
is greater than V
REF
+ 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 V
IN
,
V
OUT
, 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 V
IN
, V
OUT
, 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 supply-
ing 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.
June 1998
9
MIC2179
MIC2179
Micrel
SW
PGND
I
SENSE
Amp.
V
REF
1.245V
110m
N-channel
110m
P-channel
COMP
VIN
Error
Amp.
R
S
Q
200kHz
Oscillator
PWM
Comp.
V
OUT
L1
FB
SGND
C
C
V
IN
4.5V to 16.5V
C
IN
SYNC
C
OUT
MIC2179 [Adjustable] PWM-Mode Signal Path
Stop
13
8
17
16
D
R1
R2
7
R
C
Corrective
Ramp
Reset
Pulse
I
L1
9
10
11
12
3
1
2
19
20
V
OUT
1.245
R1
R2
1
V
SW
I
L1
Reset
Pulse
I
SENSE
I
LOAD
I
L1
Error Amp.
Output
PWM-Mode Functional Diagram
MIC2179
Micrel
MIC2179
10
June 1998
S
R
Q
One
Shot
SW
PGND
I
SENSE
Amp.
V
REF
1.245V
110m
P-channel
VIN
Skip-Mode
Comp.
I
LIMIT
Comp.
V
OUT
L1
FB
SGND
V
IN
4.5V to 16.5V
C
IN
C
OUT
MIC2179 [Adjustable] Skip-Mode Signal Path
17
16
D
R1
R2
I
LIMIT
Thresh.
Voltage
Output Control Logic
I
L1
3
1
2
19
20
9
10
11
12
7
V
OUT
1.245
R1
R2
1
V
SW
I
L1
One-Shot
Pulse
V
FB
V
REF
+ 5mV
V
REF
5mV
0
I
LIM
0
V
OUT
V
IN
Skip-Mode Functional Diagram
June 1998
11
MIC2179
MIC2179
Micrel
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 20k
. However, to
prevent feedback error due to input bias current at the FB pin,
R2 should not be greater than 100k
. After selecting R2,
calculate R1 with the following formula:
R1 = R2
V
1.245V
OUT








1
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 V
IN
, and the voltage rating for an electrolytic
should be 40% higher than V
IN.
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:
I
=
I
RMS(max)
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 0.2in, 5mm). Also, place a 0.1
F 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:
L
= V
3.0 H/V
MIN
OUT
In general, a value at least 20% greater than L
MIN
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:
I
= I
I
2
L(peak)
LOAD(max)
L(max)
+
Where:
I
= V
1
V
V
5 s
L
L(max)
OUT
OUT
IN(max)
-
To maximize efficiency, the inductor's resistance must be
less than the output switch on-resistance (preferably,
50m
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
(V
RIPPLE
) which is generated by
I
L
ESR. Therefore, ESR
must be equal or less than a maximum value calculated for a
specified V
RIPPLE
(typically less than 1% of the output volt-
age) and
I
L(max)
:
ESR
=
V
I
MAX
RIPPLE
L(max)
Typically, capacitors in the range of 100 to 220
F 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 tempera-
tures below 0
C. The voltage rating of a tantalum capacitor
must be 2
V
OUT
, and the voltage rating of an electrolytic
must be 1.4
V
OUT
.
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 P-
channel off time. The correct diode for both of these condi-
tions is a 1A diode with a reverse voltage rating greater than
V
IN
. It must be a schottky or ultrafast-recovery diode
(t
R
< 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 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 to 10nF for the capacitor and 5k
to 20k
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 compo-
nent 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.
MIC2179
Micrel
MIC2179
12
June 1998
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
AVX Corp.
General Instruments (GI)
Siliconix
1102 Silver Lake Rd.
801 17th Ave. South
10 Melville Park Rd.
2201 Laurelwood Rd.
Cary, IL 60013
Myrtle Beach, SC 29577
Melville, NY 11747
Santa Clara, CA 96056
tel: (708) 639-2361
tel: (803) 448-9411
tel: (516) 847-3222
tel: (800) 554-5565
fax: (708) 639-1469
fax: (803) 448-1943
fax: (516) 847-3150
Coiltronics
Sanyo Video Components Corp.
International Rectifier Corp.
6000 Park of Commerce Blvd.
2001 Sanyo Ave.
233 Kansas St.
Boca Raton, FL 33487
San Diego, CA 92173
El Segundo, CA 90245
tel: (407) 241-7876
tel: (619) 661-6835
tel: (310) 322-3331
fax: (407) 241-9339
fax: (619) 661-1055
fax: (310) 322-3332
Bi Technologies
Sprague Electric
Motorola Inc.
4200 Bonita Place
Lower Main St.
MS 56-126
Fullerton, CA
60005 Sanford, ME 04073
3102 North 56th St.
tel: (714) 447-2345
tel: (207) 324-4140
Phoenix, AZ 85018
fax: (714) 447-2500
tel: (602) 244-3576
fax: (602) 244-4015
June 1998
13
MIC2179
MIC2179
Micrel
Package Information
2.00 (0.079)
1.73 (0.068)
0.21 (0.008)
0.05 (0.002)
COPLANARITY:
0.10 (0.004) MAX
1.25 (0.049) REF
0.65 (0.0260)
BSC
0.875 (0.034)
REF
10
4
0
8
5.40 (0.213)
5.20 (0.205)
7.90 (0.311)
7.65 (0.301)
7.33 (0.289)
7.07 (0.278)
0.38 (0.015)
0.25 (0.010)
0.22 (0.009)
0.13 (0.005)
0.95 (0.037)
0.55 (0.022)
DIMENSIONS:
MM (INCH)
20-lead SSOP (SM)
MIC2179
Micrel
MIC2179
14
June 1998
June 1998
15
MIC2179
MIC2179
Micrel
MIC2179
Micrel
MIC2179
16
June 1998
MICREL INC.
1849 FORTUNE DRIVE
SAN JOSE, CA 95131
USA
TEL
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
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