ST AN4086 Application note
AN4086
Application note
Buck voltage regulator using the PM8903
By David Toland
Introduction
The PM8903 is a compact, high-efficiency, monolithic step-down switching voltage regulator
which can deliver up to 3 A of continuous current. The IC minimizes external components
and board space by incorporating low-resistance MOSFETs into the IC. It is used in
applications including CPU, DSP and FPGA power supplies, distributed power supplies, and
for general DC/DC converters. The following features are incorporated:
■ Input voltage range of 2.8 V to 6 V
■ Adjustable output voltage to as low as 0.6 V
■ PSKIP mode for optimizing efficiency at light load
■ Undervoltage, overvoltage, overcurrent, and overtemperature protection
■ Power Good output
■ 1.1 MHz switching frequency which enables the use of a small inductor
■ Low quiescent current when shut down (<15 µA)
■ Interleaving synchronization (up to two ICs)
■ Small VFQFPN16, 3x3 mm package
June 2012 Doc ID 023053 Rev 1 1/11
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Contents AN4086
Contents
1 Circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2/11 Doc ID 023053 Rev 1
AN4086 List of figures
List of figures
Figure 1. PM8903 schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 2. R/C snubber circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 3. PM8903 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 4. PM8903 demonstration board efficiency with V
FSW = 1.1 MHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 5. Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 6. V
, VIN, IIN ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
OUT
Figure 7. Transient load (0 A to 1.5 A). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 8. Transient load (1.5 A to 3 A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 9. Duty cycle jitter at 3 A load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 10. V
, VIN, IIN ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
OUT
Figure 11. Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 12. Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
= 3.3 V, V
IN
= 1.5 V, and
OUT
Doc ID 023053 Rev 1 3/11
Circuit description AN4086
R
OS = RFB
* V
REF
/ (V
OUT
– V
REF
)
1 Circuit description
● Output voltage setting
In Equation 1 below, the output voltage is programmed by R
formula:
Equation 1
and RFB using the
OS
where V
is 0.6 V and RFB is selected to obtain the desired regulator bandwidth (see
REF
section 6.1 of datasheet for details).
● Inductor selection
Choosing an inductor involves a compromise between dynamic response, efficiency,
cost and size. A higher inductor value will decrease the output voltage ripple, but will
increase the regulator response time to load changes.
The inductance has to be calculated to keep the ripple current (ΔI
) between 20% and
L
30% of the maximum output current, using the following equation:
Equation 2
where F
is the switching frequency, VIN is the input voltage, and V
SW
is the output
OUT
voltage.
● Output capacitor selection
The output capacitor bank will define the ripple voltage and affect the transient
response of the regulator.
During steady state operation, the output voltage ripple is affected by the ESR and the
capacitance value according to the following equations:
Equation 3
Equation 4
where ΔI
4/11 Doc ID 023053 Rev 1
is the inductor current ripple.
L
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