Datasheet

LOAD
R
O
V
3
C
2
R
1
R
FB
V
=
1
(R
Load
C
OUT
)2S
F
RCP-
10 kHz5 kHz
o
=
=
1
( )
R
2
xC
f
2S
F
CFZERO-
REF
V
¨
¨
©
§
OUT
V
=
2
R
1
¸
¸
¹
·
- x
1
R
LMR62421
www.ti.com SNVS734B –OCTOBER 2011–REVISED APRIL 2013
The simplest method to determine the compensation component value is as follows.
Set the output voltage with the following equation.
where
• R1 is the bottom resistor
and
• R2 is the resistor tied to the output voltage. (10)
The next step is to calculate the value of C3. The internal compensation has been designed so that when a zero
is added between 5 kHz & 10 kHz the converter will have good transient response with plenty of phase margin
for all input & output voltage combinations.
(11)
Lower output voltages will have the zero set closer to 10 kHz, and higher output voltages will usually have the
zero set closer to 5 kHz. It is always recommended to obtain a Gain/Phase plot for your actual application. One
could refer to the Typical Appplication section to obtain examples of working applications and the associated
component values.
Pole @ origin due to internal gm amplifier:
F
P-ORIGIN
(12)
Pole due to output load and capacitor:
(13)
This equation only determines the frequency of the pole for perfect current mode control (CMC). Therefore, it
doesn’t take into account the additional internal artificial ramp that is added to the current signal for stability
reasons. By adding artificial ramp, you begin to move away from CMC to voltage mode control (VMC). The
artifact is that the pole due to the output load and output capacitor will actually be slightly higher in frequency
than calculated. In this example it is calculated at 650 Hz, but in reality it is around 1 kHz.
The zero created with capacitor C3 & resistor R2:
Figure 19. Setting External Pole-Zero
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