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Converter Loop Design and Stability
Design Procedure Quick Steps
Setting the Output Voltage and Selecting the Feedforward Capacitor
V
out
 1.146 V 

1 
R1
R2

R1
430 kΩ
SW
SW
FB1
SUP
C2+
C2−/MODE
D1
C8
6.8 pF
C4
22 µF
C2
0.22 µF
R2
56 kΩ
V
O
1
Up to 10 V/150 mA
ƒ
z

1
2    C8  R1
TPS65100 , TPS65101
TPS65105
SLVS496C – SEPTEMBER 2003 – REVISED APRIL 2006
The TPS6510x series converter loop can be externally compensated and allows access to the internal
transconductance error amplifier output at the COMP pin. A small feedforward capacitor across the upper
feedback resistor divider speeds up the circuit as well. To test the converter stability and load transient
performance of the converter, a load step from 50 mA to 250 mA is applied, and the output voltage of the
converter is monitored. Applying load steps to the converter output is a good tool to judge the stability of such a
boost converter.
1. Select the feedback resistor divider to set the output voltage.
2. Select the feedforward capacitor to place a zero at 50 kHz.
3. Select the compensation capacitor on pin COMP. The smaller the value, the higher the low frequency gain.
4. Use a 50-k Ω potentiometer in series to C
c
and monitor V
out
during load transients. Fine tune the load
transient by adjusting the potentiometer. Select a resistor value that comes closest to the potentiometer
resistor value. This needs to be done at the highest V
in
and highest load current since the stability is most
critical at these conditions.
The output voltage is set by the external resistor divider and is calculated as:
Across the upper resistor a bypass capacitor is required to speed up the circuit during load transients as shown
in Figure 15 .
Figure 15. Feedforward Capacitor
Together with R1 the bypass capacitor C8 sets a zero in the control loop at approximately 50 kHz:
A value closest to the calculated value should be used. Larger feedforward capacitor values reduce the load
regulation of the converter and cause load steps as shown in Figure 16 .
15
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