CMOS simple buck voltage
regulators feature faster rise/fall time, faster response to fault
conditions, and improved efficiency at light loads.
Description
The MIC457x switching regulator is basically a PWM (pulse
width modulation) controller IC with a fixed gain error ampli-
fier, a 200kHz oscillator, and internal compensation network.
The non-inverting side of the error amplifier is tied to a 1.23V
bandgap reference.
200kHz
Oscillator
1.23V
Bandgap
Switch
Thermal
Shutdown
Current
Limit
Driver
Internal
Regulator
FB
SW
GND
V
IN
+7V to V
IN
(Max)
C
IN
C
OUT
L1
D1
Shutdown
Enable
SHDN
V
OUT
MIC457x-x.x
R1*
R2*
Com-
parator
Error
Amp.
* R1 and R2 external
on adjustable versions
Figure 1. Block Diagram (Fixed Version)
Buck Regulator Design Procedure
Select the MIC4574 (0.5A), MIC4575 (1A), or MIC4576 (3A)
based on the required output current. If higher current rated
regulators are chosen for low current applications, make sure
the current limit range is appropriate for that application.
Output Voltage
For fixed output voltages, 3.3V or 5.0V versions are available.
The output voltage of the adjustable regulators is configured
using an external resistive divider.
V
1.23V 1 R2
R1
OUT
=
+






For best performance, R1 should be between 1k and 10k.
Inductor Selection Criteria
The following criteria is used for inductor selection:
Mode of operation (continuous or discontinuous).
Peak inductor current
Volt-seconds (V·s) applied to the inductor
Definitions
Critical Inductance Condition The critical inductance condi-
tion is when the current through the inductor decays to zero
just prior to the next on time of the regulator switch. This
occurs at the boundary between continuous and discon-
tinuous operation.
Discontinuous Operation Discontinuous operation occurs
when, for any condition of input voltage or output current,
the inductor current decays to zero before the next on
time of the regulator switch.
Continuous Operation Continuous operation occurs when,
for any condition of input voltage or output current, the
inductor current does not decay to zero before the next on
time of the regulator switch.
Continuous Conduction Operation
Critical Inductance
Compute the value of critical inductance required for the
application at the worst case combination of input voltage and
output load current. This will be the minimum value of
inductance that will guarantee continuous conduction opera-
tion over all input voltage and output load conditions.
At the critical inductance condition, the peak inductor current
is twice the average current. The average current is the
current delivered to the load. The peak current at the critical
inductance condition is:
(1)
I
D V
V
L f
PEAK
IN
OUT
S
= (
)
Where:
D = duty cycle
D = switch on time/switch cycle time, T
ON
/
= switch cycle time, 1 / f
S
, (s)
V
IN
= input (supply) voltage (V)
V
OUT
= regulator output voltage (V)
L = inductance of filter inductor (H)
f
S
= switching frequency (Hz)
The input power will be assumed to be equal to the output
power.
(2)
E

V
I D
V
R
FF
IN L
OUT
2
LOAD
=
Where:
E
FF
= estimated efficiency
reasonable initial estimate 80% (0.8)
R
LOAD
= load resistance ( )
Micrel, Inc. 1849 Fortune Drive San Jose, CA 95131 USA tel + 1 (408) 944-0800 fax + 1 (408) 944-0970 http://www.micrel.com Application Note 14
Micrel
Application Note 14
2
March 2001
and,
(3)
L
R

1 D
2 f
CRITICAL
LOAD
S
= (
)
Duty Cycle
Compute the duty cycle required at the maximum required
input voltage and minimum load current. If your cannot
guarantee a minimum load current, an additional resistive
load may be required at the regulator output.
D
V
V
MIN
OUT
IN(max)
=
Use this value of D
MIN
and the minimum value of R
LOAD
in
equation (3) to determine the value of critical inductance.
This is the minimum value of inductance required. Changing
the minimum load and/or the maximum input voltage require-
ment changes the minimum required critical inductance.
The value of inductance can be chosen to allow the regulator
to operate in discontinuous mode under certain conditions.
Discontinuous mode typically occurs at maximum input and
minium load current. In many cases this may not present a
problem, however, it should be verified that operation in
discontinuous mode still allows the circuit to satisfy the load
regulation requirement.
Maximum V·s
Compute the maximum volt-microseconds applied to the
inductor:
V s
V
V
V
V
IN
OUT
OUT
IN(max)
= (
) Inductor Peak Current
Compute the peak current through the inductor. This is the
sum of the maximum load current and peak ripple current
though the inductor.
I
1
2
V
V
L
V
V
V
R
PEAK
IN(max)
OUT
OUT
IN(max)
OUT
LOAD
=





+ Inductor Selection
Refer to the Inductor Selection and Cross Reference table
to select the appropriate inductor for your application. The
selection should satisfy the following:
Inductance > Calculated Critical Inductance
Volt-second Capability > Calculated V·
µ
s
(if applicable)
I
DC
> Calculated I
PEAK
Current
×
0.85
Output Capacitor Selection
For stable operation, the output capacitor must satisfy the
following:
C
13300 V
V

L
OUT
IN(max)
OUT





Where:
C
OUT
= output capacitance (
µ
F)
L = inductance (
µ
H)
This guarantees that the dominant pole pair of the LC filter
does not occur at a frequency that is too high for the
regulators internal loop compensation circuitry. This compu-
tation may result in a capacitor value that is too small to
provide adequate peak-to-peak output ripple reduction.
Peak-to-peak ripple voltage is a function of the capacitor
value and type. A low ESR/ESL (equivalent series resis-
tance/equivalent series inductance) capacitor should be used
for lower ripple voltage. (Standard capacitors may be paral-
leled to reduce the effective ESR/ESL value.) Low ESR
electrolytic capacitors are available from Panasonic, Nichicon,
and United Chemicon.
Maximum peak-to-peak ripple voltage (assuming no ESR or
ESL in the filter capacitor) can be estimated as follows:
V
1
C
V


V
L
1
2
V
V
P-P
IN(max)
OUT
OUT
2
IN
2
=
× (
)
× ×
2 Input Capacitor Selection
The input bypass capacitor must be at least 47
µ
F to maintain
stability. Low ESR capacitors are recommended. If the
operating temperature range is below 25
°
C, the value of this
capacitor should be increased. Adding a ceramic or solid
tantalum capacitor near the input pin will also increase
regulator stability at low temperatures. The capacitors ripple
current rating should be more than the ripple component of
the inductor current:
I
2
V
V
L
RIPPLE
IN(max)
OUT
=




Catch Diode Selection
Although either a Schottky or a fast recovery diode can be
used, a Schottky diode will provide the best performance
because its lower voltage drop and faster switching speed will
result in higher efficiency. Fast recovery diodes with abrupt
turn-off characteristics may cause EMI problems and/or
instabilities.
The reverse voltage rating of the catch diode should be at
least 1.25
×
the maximum input voltage.
Standard 1N400x series diodes should not be used. The
reverse recovery time of this type of diode is excessive which
will cause additional noise and heat dissipation in the diode
and the regulators internal power switch. March 2001
3
Application Note 14
Application Note 14
Micrel
Typical Applications
Fixed 3.3V Buck Regulator
Figure 2 shows a 3.3V buck regulator using inexpensive
standard components.
The high efficiency (~80%) and low form factor afforded by
the use of a new TO-263 surface mount package makes this
ideal for battery operated designs.
1N5822
220µF
47µF
ON/OFF
GND
FEEDBACK
OUTPUT
INPUT
60µH
MIC4575-
3.3
+4.5V to +24V
Unregulated
Input
3.3V, 1A
Output
Figure 2. 3.3V Buck Regulator
Figure 3. Undervoltage Lockout
ON/OFF
GND
FEEDBACK
OUTPUT
INPUT
MIC4575
Unregulated
DC Input
R1
R2
Z1
20k
Q1
If lower ripple voltage is desired, the standard 220
µ
F capaci-
tor can be replaced with a standard 330
µ
F. For lower ripple
at a small size, an Oscon 105A220M capacitor (220
µ
F, 35m ESR) can be used. Application Note 14
Micrel
Application Note 14
4
March 2001
Inductor Selection and Cross Reference
Renco Part
1
I
PC
V·
µ
s
L
Description
Part No.
(A)
(V·
µ
s)
(
µ
H)
RL5341-20-1
1
43
20
powdered iron
RL5341-48-1
1
51
48
RL5341-68-1
1
155
68
RL5341-100-1
1
200
100
RL5341-150-1
1
330
150
RL5341-220-1
1
400
220
RL5341-330-1
1
680
330
RL5341-470-1
1
796
470
RL5341-680-1
1
1500
680
RL5341-1000-1
1
2000
1000
RL5342-20-1
1
26
20
moly permalloy
RL5342-48-1
1
60
48
RL5342-68
1
88
68
RL5342-100-1
1
116
100
RL5342-150-1
1
193
150
RL5342-220-1
1
285
220
RL5342-330-1
1
400
470
RL5342-470-1
1
604
470
RL5342-680-1
1
888
680
RL5342-1000-1
1
1200
1000
RL5341-20-3
3
140
20
powdered iron
RL5341-48-3
3
257
48
RL5341-68-3
3
471
68
RL5341-100-3
3
640
100
RL5341-150-3
3
885
150
RL5341-220-3
3
1272
220
RL5341-330-3
3
2155
330
RL5341-470-3
3
3221
470
RL5341-680-3
3
4784
680
RL5341-1000-3
3
6000
1000
RL5342-20-3
3
81
20
moly permalloy
RL5342-48-3
3
177
48
RL5342-68-3
3
273
68
RL5342-100-3
3
392
100
RL5342-150-3
3
591
150
RL5342-220-3
3
872
220
RL5342-330-3
3
1202
470
RL5342-470-3
3
1946
470
RL5342-680-3
3
2837
680
RL5342-1000-3
3
3900
1000
1. Renco Electronics Inc., Deer Park, New York; tel: (516) 586-5566
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 grant