Datasheet
F
P2
=
1
C
COMP
x C
HF
C
COMP
+ C
HF
2S x R
COMP
x
¹
·
©
§
[Hz]
2Sx R
COMP
x C
COMP
1
F
Z
=
[Hz]
F
P1
= 0 [Hz]
V
OUT
COMP
FB
LM25117
+
-
R
FB2
R
FB1
R
COMP
C
COMP
C
HF
REF
Error
Amplifier
Type 2 Compensation
Components
-+
PWM
Comparator
+
-
RAMP Generator
Output
(optional)
L
O
R
RAMP
x C
RAMP
x R
S
x A
S
K =
LM25117
LM25117-Q1
www.ti.com
SNVS714E –APRIL 2011–REVISED MARCH 2013
The positive slope inductor current ramp is emulated by C
RAMP
connected between RAMP and AGND and R
RAMP
connected between SW and RAMP. R
RAMP
should not be connected to VIN directly because the RAMP pin
absolute maximum voltage rating could be exceeded under high input voltage conditions. C
RAMP
is discharged by
an internal switch during the off-time and must be fully discharged during the minimum off-time. This limits the
ramp capacitor to be less than 2nF. A good quality, thermally stable ceramic capacitor is recommended for
C
RAMP
.
The selection of R
RAMP
and C
RAMP
can be simplified by adopting a K factor, which is defined as:
(4)
Where A
S
is the current sense amplifier gain which is normally 10. By choosing 1 as the K factor, the regulator
removes any error after one switching cycle and the design procedure is simplified. See Application Information
for detailed information.
Error Amplifier and PWM Comparator
The internal high-gain error amplifier generates an error signal proportional to the difference between the FB pin
voltage and the internal precision 0.8V reference. The output of error amplifier is connected to the COMP pin
allowing the user to provide Type 2 loop compensation components, R
COMP
, C
COMP
and optional C
HF
.
Figure 25. Feedback Configuration and PWM Comparator
R
COMP
, C
COMP
and C
HF
configure the error amplifier gain and phase characteristics to achieve a stable voltage
loop gain. This network creates a pole at DC (F
P1
), a mid-band zero (F
Z
) for phase boost, and a high frequency
pole (F
P2
). The recommended range of R
COMP
is 2kΩ to 40kΩ. See Application Information for detailed
information.
(5)
(6)
(7)
The PWM comparator compares the emulated current sense signal from Ramp Generator to the voltage at the
COMP pin through a 1.2V internal voltage drop and terminates the present cycle when the emulated current
sense signal is greater than V
COMP
- 1.2V.
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