Datasheet

d(SP)
CS
T 70ns
4 V / 2
C
40 4 V
+
æ ö
< ´
ç ÷
W
è ø
CS
CHARGE
CS SW
CHARGE PK
1
R
V
2 ln C f
V V
=
æ ö
´ ´ ´
ç ÷
-
è ø
PK
MAX
1.4 V
V
D
=
BLANK
C
CS
R
CS
VREF
RAMP/CS
GND
UCC28250
BLANK
C
CS
R
CS
RAMP/CS
GND
UCC28250
V
IN
36 V to 75 V
UCC28250
www.ti.com SLUSA29C –APRIL 2010– REVISED JULY 2011
RAMP/CS (PWM Ramp Input or Current Sense Input) (16/3)
The UCC28250 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.
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 11. The internal pull-down switch has approximately 40-Ω on-resistance. The RAMP/CS pin is clamped
internally to 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.
(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.
(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.
Choose the ramp amplitude to accommodate the voltage range of the COMP pin and the maximum duty cycle
required by the power stage. Use the following equation to select V
PK
, in the equation, D
MAX
is the maximum duty
cycle for primary outputs.
(6)
Figure 11. Fixed Ramp Generation/Ramp Generation With Input Voltage Feedforward
Voltage feed-forward can be achieved by driving R
CS
from line input VIN. The peak of RAMP/CS is proportional
to VIN and output has have much faster line transient response. When the UCC28250 is used for the
primary-side control, RAMP parameters are critical for the optimal pre-biased start up performance. Refer to the
‘Voltage Mode Control and Input Voltage Feed-Forward’ section of the Functional Description section for a
detailed design procedure of choosing R
CS
.
If the line input cannot be easily accessed due to limited board area or other limitation, a RAMP signal with fixed
peak voltage can be implemented by simply driving R
CS
from 3.3 V VREF (Figure 11).
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