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| SP6123 Datasheet, PDF (13/18 Pages) Sipex Corporation – Low Voltage, Synchronous Step-Down PWM Controller Ideal for 2A to 10A, Small Footprint, DC-DC Power Converters | |||
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1
fP(LC) =
2Ïâ LCOUT
The open loop gain of the whole system can be
divided into the gain of the error amplifier,
PWM modulator, buck converter, and feedback
resistor divider. In order to crossover at the
selected frequency fco, the gain of the error
amplifier has to compensate for the attenuation
caused by the rest of the loop at this frequency.
In the RC network shown in Figure 1, the product
of R1 and the error amplifier transconductance
determines this gain. Therefore, R1 can be de-
termined from the following equation that takes
into account the typical error amplifier
transconductance, reference voltage and PWM
ramp built into the SP6123.
R1
=
2083VOUT fCO fZ(ESR)
VIN fP(LC)2
In Figure 1, R1 and C1 provides a zero fZ1 which
needs to be placed at or below fP(LC). If fZ1 is
made equal to fP(LC) for convenience, the value
of C1 can be calculated as
C1
=
1
2ÏfP(LC)R1
The optional C2 generates a pole fP1 with R1 to
cut down high frequency noise for reliable op-
eration. This pole should be placed one decade
higher than the crossover frequency to avoid
APPLICATIONS INFORMATION
erosion of phase margin. Therefore, the value of
the C2 can be derived from
C2
=
1
20ÏfCOR1
Figure 2 illustrates the overall loop frequency
response and frequency of each pole and zero.
To fine-tune the compensation, it is necessary to
physically measure the frequency response us-
ing a network analyzer.
Gain
-20db/dec
-40db/dec
Loop
-20db/dec
f
-20db/dec
Error Amplifier
fZ1 fP(LC) fZ(ESR) fCO fP1
-20db/dec
f
Figure 2. Frequency response of a stable system and its
error amplifier.
3V to 5.5V
R1
5.0
CB
2.2µF
CP
47pF
1 GL
2 VCC
3 GND
SP6123A
U1
4 COMP
BST 8
GH 7
SWN 6
VFB 5
RZ
CZ
20k
680pF
D1
MBR0530
CBST
1µF
CIN
47µF Ceramic
Q1
FDS6890A
L1
Q1
FDS6890A
1µH
D2
STPS2L25U
1.6V/4A
R2
C1
COUT
80k
330pF
4x47µF Ceramic
R3
80k
Figure 3. SP6123 Buck converter design with ceramic output capacitors.
Date: 5/25/04
SP6123 Low Voltage, Synchronous Step Down PWM Controller
13
© Copyright 2004 Sipex Corporation
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