Datasheet

R
CS
GATE
CS
RTN
R
S
C
S
ARTN
COM
SLOPE
SLOPE_D
MAX
S
SL_EX
V (mV)
V (mV)
D
R ( ) = 1000
I ( A)
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W ´
m
FRS
R
FRS
47pF
Synchronization
Pulse
V
SYNC
T
SYNC
1:1
1000pF
R
T
RTN
FRS
R
FRS
47pF
Synchronization
Pulse
V
SYNC
T
SYNC
ARTN
COM
RTN
ARTN
COM
TPS23757
SLVS948D –JULY 2009–REVISED NOVEMBER 2013 www.ti.com
Figure 25. Synchronization
Current Slope Compensation
The TPS23757 provides a fixed internal compensation ramp that suffices for most applications. R
S
(see
Figure 26) may be used if the internally provided slope compensation is not enough.
Most current-mode control papers and application notes define the slope values in terms of V
PP
/t
S
(peak ramp
voltage / switching period), however the electrical characteristics table specifies the slope peak (V
SLOPE
) based
on the maximum (78%) duty cycle. Assuming that the desired slope, V
SLOPE-D
(in mV/period), is based on the full
period, compute R
S
per the following equation where V
SLOPE
, D
MAX
, and I
SL-EX
are from the electrical
characteristics table with voltages in mV, current in μA, and the duty cycle is unitless (e.g., D
MAX
= 0.78).
(6)
Figure 26. Additional Slope Compensation
C
S
may be required if the presence of R
S
causes increased noise, due to adjacent signals like the gate drive, to
appear at the C
S
pin.
Blanking Period, R
BLNK
Selection of the blanking period is often empirical because it is affected by parasitics and thermal effects of every
device between the gate-driver and output capacitors. The minimum blanking period prevents the current limit
and PWM comparators from being falsely triggered by the inherent current spike that occurs when the switching
MOSFET turns on. The maximum blanking period is bounded by the output rectifier's ability to withstand the
currents experienced during a converter output short, and the minimum duty cycle required.
If blanking beyond the internal default is desired choose R
BLNK
using R
BLNK
(kΩ) = t
BLNK
(ns).
1. For a 100 ns blanking interval
(a) R
BLNK
(kΩ) = 100
(b) Choose R
BLNK
= 100 kΩ.
The blanking interval can also be chosen as a percentage of the switching period.
1. Compute R
BLNK
as follows for 2% blanking interval in a switcher running at 250 kHz.
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