Datasheet

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SLUS542F − OCTOBER 2003 − REVISED JULY 2009
21
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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
LINEUV
+ V
ON
R
IN2
R
IN1
) R
IN2
at turn on, and
V
LINEUV
+
ǒ
V
OFF
* V
VON
R
IN1
) I
HYST
Ǔ
R
IN2
at turn off.
Since V
ON
and V
OFF
are given by the power supply specification, V
LINEUV
equals the 1.27-V threshold of the
line monitor and I
HYST
is already defined as:
I
HYST
+
V
REF
2
1
R
DEL
0.05
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
+
ǒ
V
ON
* V
OFF
Ǔ
I
HYST
R
IN2
+ R
IN1
1.27 V
V
ON
* 1.27 V
Similar methods can be used to define the divider components of the overvoltage protection input of the
UCC2892 and UCC2894 controllers.
Step 6. Current Sense and Slope Compensation
The UCC2891 family 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
+
1
2p f
F
C
F
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
ǒ
dV
L
ńdt
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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