UCC2897_15 Datasheet, PDF (21/40 Pages) Texas Instruments – ADVANCED CURRENT-MODE ACTIVE CLAMP PWM CONTROLLER

UCC2897
ą
SLUS591G − NOVEMBER 2003 − REVISED DECEMBER 2008
APPLICATION INFORMATION: SETUP GUIDE
Step 5. Input Voltage Monitoring
The input voltage monitoring functions is governed by the following two expressions of the voltage at the
LINEUV terminal (pin 15):
V_VON + V_ON
R_IN2
R_IN1 ) R_IN2
at turn on, and
(18)
ǒ Ǔ V_VON +
V_OFF * V_VON
R_IN1
)
I_HYST
R_IN2 at turn off.
(19)
Since V_ON and V_OFF are given by the power supply specification, V_VON equals the 1.27-V threshold of the line
monitor and I_HYST is already defined as:
I_HYST
+
2.5 V
R_DEL
0.05
(20)
the two unknown, R_IN1 and R_IN2 are fully determined. Solving the equations results the following two
expressions for the input voltage divider:
R_IN1
+
V_ON * V_OFF
I_HYST
(21)
R_IN2 + R_IN1
1.27 V
V_ON * 1.27 V
(22)
Similar methods can be used to define the divider components of the overvoltage protection input of the
UCC2897 controller.
Step 6. Current Sense and Slope Compensation
The UCC2897 offers onboard, user programmable slope compensation. The programming of the right 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 (R_F) 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 (pin 7) and the GND terminal (pin 6). While the value of C_F can be freely selected as the first step
of the filter design, it should be 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
can be calculated from the filter capacitance and the desired filter corner frequency f_F.
R_F + _2p
1
f_F
C_F
(23)
After R_F is defined R_SLOPE can be 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 ^ǒdV_Lńdt^Ǔ, m equals 1. The minimum value of m is 0.5
to prevent current loop instability. Best current mode performance can be achieved around m=1. The further
increase of m moves the control closer to voltage mode control operation.
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