UCC28251_15 Datasheet, PDF (20/55 Pages) Texas Instruments – UCC28251 Advanced PWM Controller With Prebias Operation

UCC28251
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
Feature Description (continued)
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PS DEAD TIME
vs
PS RESISTOR
400
350
300
250
200
150
100
50
0
0 20 40 60 80 100 120 140 160 180 200 220 240
RPS - PS Resistor - kW
Figure 28. Dead Time T_D(ps) vs. External Resistor R_PS at PS Pin
8.3.7 RAMP/CS (PWM Ramp Input or Current Sense Input) (16/3)
The UCC28251 can be controlled using either voltage mode or current mode. RAMP/CS is a multi-function pin
used either to generate the ramp signal for voltage mode control or to sense current for current mode control.The
following sections describe the RAMP/CS functionality for voltage mode and current mode control.
8.3.7.1 RAMP: Voltage Mode Control With Feed-Forward Operation
For voltage mode control, a resistor R_CS and a capacitor C_CS must be connected to the RAMP/CS pin as shown
in Figure 29. The internal pull-down switch has approximately 40-Ω on-resistance. The RAMP/CS pin is clamped
internally to not exceed 4 V for internal device protection. The C_CS value must be small enough to discharge the
RAMP/CS pin from its peak voltage to ground within the pulse width of the BLANK signal (T_D(sp) + 70 ns). The
following formula derives a C_CS value.
C_CS
<
æ
ç
è
4V/2
40 W
ö
÷
´
ø
T_D(SP) + 70 ns
4V
(4)
A C_CS value less than 650 pF works for most applications. In order to minimize the impacts of parasitic
capacitance caused by the PCB layout and routing, a minimum of 100 pF is recommended for C_CS. Once C_CS is
determined, R_CS can be calculated according to the desired ramp peak amplitude.
R=
1
CS
2
´
ln
æ
ç
è
V
CHARGE
V
-V
CHARGE
PK
ö
÷
ø
´
C
CS
´
f
SW
(5)
In this equation, the V_CHARGE is the voltage used to generate the ramp, V_PK is the desired ramp amplitude and
the f_SW is the switching frequency.
20
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