Datasheet

RAMP
10 nF
LO
-VCC
C
RAMP
R
RAMP
10 nF
1N914
R
RAMP
=
VCC ± 0.5V + V
RAMP
25 PA - I
OS
RAMP
VCC
C
RAMP
R
RAMP
R
RAMP
=
VCC - V
RAMP
I
OS
- 25 PA
V
RAMP
=
((V
IN
± V
OUT
) x g
m
+ I
OS
) x T
V
OUT
V
IN
C
RAMP
x
R
S
=
V
CS(TH)
I
OUT
+
V
OUT
x T
L
C
RAMP
=
I
OS
x L
V
OUT
x A x R
S
C
RAMP
=
g
m
x L
A x R
S
x
1 +
5 - V
OUT
V
IN(MIN)
LM25116
SNVS509D –APRIL 2007–REVISED FEBRUARY 2013 www.ti.com
(39)
Best Performance Method:
This minimizes the current limit deviation due to changes in line voltage, while maintaining near optimal slope
compensation.
Calculate optimal slope current, I
OS
= (V
OUT
/ 3) x 10 µA/V. For example, at V
OUT
= 7.5V, I
OS
= 25 µA.
(40)
Calculate V
RAMP
at the nominal input voltage.
(41)
For V
OUT
> 7.5V, install a resistor from the RAMP pin to VCC.
(42)
Figure 39. R
RAMP
to VCC for V
OUT
> 7.5V
For V
OUT
< 7.5V, a negative VCC is required. This can be made with a simple charge pump from the LO gate
output. Install a resistor from the RAMP pin to the negative VCC.
(43)
Figure 40. R
RAMP
to -VCC for V
OUT
< 7.5V
If a large variation is expected in VCC, say for V
IN
< 11V, a Zener regulator may be added to supply a constant
voltage for R
RAMP
.
MODULATOR TRANSFER FUNCTION
The following equations can be used to calculate the control-to-output transfer function:
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