Datasheet
80
60
40
20
0
-20
-40
180
135
90
45
0
-45
-90
-135
-180
10 100 1k 10k 100k 1M
f-Frequency-Hz
Gain-dB
Phase-deg
Gain
Phase
V
V
FB
EA
f
zea
f
p-ea
P
o O
1
= (a)
× R × C
¦
p
p-f
ff
1
= (b)
2 R2 C
¦
p ´ ´
z-f
ff
1
= (c)
2 R1 C
¦
p ´ ´
TPS61093
www.ti.com
SLVS992A –SEPTEMBER 2009–REVISED NOVEMBER 2009
SMALL SIGNAL STABILITY
The TPS61093 integrates slope compensation and the RC compensation network for the internal error amplifier.
Most applications will be control loop stable if the recommended inductor and input/output capacitors are used.
For those few applications that require components outside the recommended values, the internal error
amplifier’s gain and phase are presented in Figure 14.
Figure 14. Bode Plot of Error Amplifier Gain and Phase
The RC compensation network generates a pole f
p-ea
of 57-kHz and a zero f
z-ea
of 1.9-kHz, shown in Figure 14.
Use Equation 7 to calculate the output pole, f
P
, of the boost converter. If f
P
<< f
z-ea
. due to a large capacitor
beyond 10 μF, for example, a feed forward capacitor on the resistor divider, as shown in Figure 14, is necessary
to generate an additional zero f
z-f
. to improve the loop phase margin and improve the load transient response.
The low frequency pole f
p-f
and zero f
z-f
generated by the feed forward capacitor are given by Equation 8 and
Equation 9:
(7)
(8)
(9)
Where C
ff
= the feed-forward capacitor.
For example, in the typical application circuitry (see Figure 1), the output pole f
P
is approximately 1-kHz. When
the output capacitor is increased to 100-μF, then the f
P
is reduced to 10-Hz. Therefore, a feed-forward capacitor
of 10-nF compensates for the low frequency pole.
A feed forward capacitor that sets f
z-f
near 10-kHz improves the load transient response in most applications, as
shown in Figure 8.
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Product Folder Link(s): TPS61093