UCC2897A_15 Datasheet, PDF (32/46 Pages) Texas Instruments – Advanced Current-Mode Active-Clamp PWM Controller

UCC2897A
SLUS829E – AUGUST 2008 – REVISED APRIL 2015
www.ti.com
9.2.2.6 Current Sense and Slope Compensation
The UCC2897A offers onboard user-programmable slope-compensation. The programming of the correct
amount of slope compensation is accomplished by the appropriate selection of two external resistors, R_F and
R_SLOPE.
First, the current-sense filter-resistor value (RF) must be calculated based on the desired filtering of the current-
sense signal. The filter consists of two components, C_F and R_F. The C_F filter capacitor is connected between the
CS pin and the GND pin. While the value of CF is selected freely as the first step of the filter design, the value of
CF is minimized to avoid filtering the slope compensation current exiting the CS pin. The recommended range for
the filter capacitance is between 50 pF and 270 pF. The value of the filter resistor is calculated from the filter
capacitance and the desired filter corner frequency f_F.
1
R_F = _{2p ´ fF ´ CF}
(26)
After RF is defined, RSLOPE is calculated. The amount of slope compensation is defined by the stability
requirements of the inner-peak current-loop of the control algorithm and is measured by the number m. When the
slope of the applied compensation ramp equals the down-slope of the output-inductor current waveform reflected
across the primary-side current-sense resistor (dV_L / dt), m = 1. The minimum value of m is 0.5 to prevent
current-loop instability. The best current-mode performance is achieved around m = 1. The increase of m moves
the control closer to the voltage-mode control operation.
In the UCC2897A controllers, slope compensation is implemented by sourcing a linearly-increasing current at the
CS pin. When this current passes through the current-sense filter resistor (RF), the current converts to a slope-
compensation ramp which is characterized by (dV_S / dt). The (dV_S / dt) of the slope-compensation current is
defined by R_SLOPE according to Equation 27.
dl_S
5´2 V
dt ⁼ t_ON ´ R_SLOPE
where
• 2 V is the peak-to-peak ramp amplitude of the internal oscillator waveform
• 5 is the multiplication factor of the internal current mirror
(27)
The voltage equivalent of the compensation ramp (dV_S / dt) is obtained easily by multiplying with R_F. After
introducing the application specific m and (dV_L / dt) values, Equation 28 is rearranged for R_SLOPE.
R_SLOPE
=
5 ´ 2 V ´ R_F
t_ON
´
m´
æ
çè
dV_L
dt
ö
÷ø
(28)
+V_IN
Bootstrap
Bias
C_BIAS
VIN
VDD
AUX
UCC2897A
OUT
CS
--V_IN
GND FB
C_CLAMP
Drive
Q_AUX
Connection
C_IN
Q_MAIN
Synchronous
Rectifier
Control
R_OT
R_CS
Secondary--Side
Error Amplifier
and Isolation
Figure 29. Active Clamp Forward Converter
Load
32
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