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UCC28251 Datasheet(PDF) 24 Page - Texas Instruments

Part No. UCC28251
Description  UCC28251 Advanced PWM Controller With Prebias Operation
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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UCC28251 Datasheet(HTML) 24 Page - Texas Instruments

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VSENSE
VREF
COMP
FB/EA-
REF /EA+
SS
UCC28251
+
+
C
SS
C
Z2
R
Z2
C
P1
+
C
Z3
R
Z3
R
O2
R
O1
VOUT
External
Reference
SS
SS
COMP _ final
27 A T
C
V
m
´
=
UCC28251
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
www.ti.com
Feature Description (continued)
For primary-side control, the internal error amplifier is connected as a buffer stage. In other words, the COMP pin
is shorted to the FB/EA- pin, and the output of an external error amplifier is connected to the REF/EA+ pin
through an optical coupler (Figure 33). In this case, the output start-up is an open loop soft start because the
COMP follows the soft-start voltage instead of the voltage loop output. The soft-start time is still determined by
external capacitor CSS and the 27-µA internal charge current. The voltage depends on the value of final COMP
voltage which corresponds to the regulated primary output duty cycle. According to the desired soft start time and
COMP pin voltage level at steady state, the SS pin capacitor can be calculated as:
(9)
After soft start, the voltage at SS pin is eventually clamped at around 4 V. Under fault conditions (UVLO, internal
thermal shut down, OVP/OTP, hiccup mode), or when externally disabled, SS pin is pulled down to ground
quickly by an internal switch with 2 k
Ω on resistance to prepare for re-start. Pulling SS pin to ground externally
shuts down the controller as well.
Figure 33. Error Amplifier EAMP Connections for Primary-Side Control
8.3.13 ILIM (Current Limit for Cycle-by-Cycle Over-Current Protection) (17/4)
Cycle-by-cycle current limit is accomplished using the ILIM pin for current mode control or for voltage mode
control. The input to the ILIM pin represents the primary current information. If the voltage sensed at ILIM pin
exceeds 0.5 V, the current sense comparator terminates the pulse of output OUTA or OUTB. If the high current
condition persists, the controller operates in a cycle-by-cycle current limit mode with duty cycle determined by the
current sense comparator instead of the PWM comparator. ILIM pin is pulled down by an internal switch when
OUTA or OUTB goes low. This internal switch remains on for an additional 70 ns after OUTA or OUTB goes high
to blank leading edge transient noise in the current sensing loop. This reduces the filtering requirements at the
ILIM pin and improves the current sense response time.
24
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Product Folder Links: UCC28251


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