UCC1883 Unitrode Semiconductor, UCC1883 Datasheet - Page 5

• Manufacturer Part Number: UCC1883
• Description: Micropower Peak Current Mode Controller
• Manufacturer: Unitrode Semiconductor

APPLICATION INFORMATION
UNDERVOLTAGE LOCKOUT AND SOFT START
When power is first applied to the UCC1883, CSTART is
held at VSS until VCC exceeds 9.0V and VDD exceeds
4.4V. During this period of UVLO, the following state ex-
ists:
1) CSTART
2) OUT
3) VREG
4) VLIMIT
Once adequate operating voltages have been estab-
lished, the input current limit function is enabled. CSTART
and OUT are still held low until the voltage on the VLINE
pin exceeds 1.2V, indicating ample voltage is stored on
the converter’s bulk filter capacitor. At that time, CSTART
is released and allowed to charge from an internal 25 A
current source. The UCC1883 then begins to transfer en-
ergy to the secondary side of the converter by pulse width
modulating the ramp voltage on CT against the charging
voltage on CSTART. During this soft start period, all fault
functions, pulse-by-pulse current limiting, and input cur-
rent limiting are enabled. Note that the dV/dt of CSTART
must be strictly less than the dV/dt established on the
UCC1885 SOFTREF pin, if secondary-side soft start is
utilized. The UCC1883 continues the blind soft start pro-
cedure until the first set of communication pulses is re-
is held low,
is held low,
is forced to VCC until VCC > 4V, and
becomes a high impedance output.
Figure 4. Start Up Waveforms
5
ceived from the secondary side via the isolation interface.
At that time, all control of the power switch is effectively
transferred to the secondary-side regulation IC. Should
communication pulses never be received, or should they
be discontinued during operation, blind PWM operation
continues with the output pulse width limited by the inter-
nal 50% duty cycle clamp or pulse-by-pulse current limit
with all fault processing enabled.
USER BIAS PROGRAMMING
The RBIAS pin may be used to set the amount of quies-
cent current consumed by certain analog circuits within
the UCC1883. A resistor from this pin to VSS establishes
a reference current according to the equation
IBIAS
Recommended range for RBIAS is 39.2k to 392k . In-
ternal circuits on the UCC1883 consume a total quies-
cent current of 9
amounting to about 85 A at room temperature. Additional
dynamic current consumption may be calculated with
C
quency f
PD
I
DYNAMIC
(see specifications), given a certain oscillator fre-
RBIAS
OSC
1.2V
, from the equation
C
PD
V
CC
IBIAS, plus some fixed currents
f
OSC
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