FL7734 View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
Manufacturer
FL7734
Fairchild Semiconductor
FL7734 Datasheet PDF : 13 Pages
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Short-LED Protection
In a short-LED condition, the switching MOSFET and
secondary diode are usually stressed by the high
powering current.
GATE Turn-off
VCS-HIGH-CL (1.2 V)
LEB
VCS-LOW-CL (0.2 V)
CS
VVS-LOW-CL-H (0.5 V) /
VVS-LOW-CL-L (0.4 V)
S/H
VS
Short-LED
Protection
Debounce
Count
VVS-SLP-TH (0.4 V)
Figure 21. Short LED Protection
FL7734 changes the current-limit level in a short-LED
condition. When sampled VS voltage is lower than VVS-
LOW-CL-L (0.4 V), the current-limit level is reduced to 0.2 V
from 1.2 V, as shown in Figure 21 so that powering is
limited and external components’ current stress is
relieved. When the sampled VS voltage is continuously
lower than VVS-SLP-TH (0.4 V) for 3 consecutive switching
cycles, short-LED protection is triggered with gate
shutdown. After all types of protection including short-
LED protection is triggered, FL7734 internally counts 4
seconds for auto restart and begins startup sequence
again.
Open-LED Protection
When output load is open as high impedance, the
output capacitor should be protected by limiting the
capacitor voltage less than its maximum rating. FL7734
can detect the output over-voltage condition by sensing
both VDD and VS voltages. When VDD voltage is
higher than VDD-OVP (27 V typical) or sampled VS voltage
is higher than VVS-OVP (3 V typical), protection is
triggered. The protection mode is auto restart so normal
operation resumes when the fault condition is removed.
Sensing Resistor Short Protection
During SS3, the controller operates in current-mode
control and the peak CS voltage is 0.2 V during
switching mode. When a sensing resistor is short
circuited, CS voltage cannot reach 0.2 V and turn on
time is maximized with potential damage of switching
MOSFET. In order to provide protection against the
failure, FL7734 compares CS voltage with VCS-SRSP
(0.1 V) during the initial two switching operation. When
VCS doesn’t reach 0.1 V for the two switching, Sensing
Resistor Short Protection (SRSP) is triggered. In normal
condition, input voltage corresponding to 1.5 V VVIN is
high enough to make VCS higher than 0.1 V with turn-on
time shorter than maximum turn-on time.
SS2
SS3
VVIN
1.5 V
VGATE
VCS
0.1 V
Max. tON
SRSP
Time
Figure 22. Sensing Resistor Short Protection
Under-Voltage Lockout (UVLO)
The turn-on and turn-off thresholds are fixed internally at
10.6 V and 7.75 V, respectively. During startup, the
VDD capacitor must be charged higher than 10.6 V
through the external bleeding MOSFET. The bleeding
MOSFET supplies VDD operating current until power
can be delivered from the auxiliary winding of the main
transformer. Generally at small phase angle range, VDD
supply time from auxiliary winding is short and VDD could
reach to VDD-OFF (7.75 V). If VDD drops below VDD-OFF,
VDD hiccup occurs with a certain hiccup frequency
determined by VDD capacitor value and VDD supply
current from auxiliary winding. This hiccup mode could
cause LED flicker. In order to remove the unstable
mode of operation, external bleeding circuit never allows
VDD voltage to fall down less than VDD-OFF (7.75 V) once
input power is supplied.
Over-Temperature Protection (OTP)
The built-in temperature-sensing circuit shuts down
PWM output if the junction temperature exceeds 150°C.
After Over-Temperature Protection (OTP) is triggered,
FL7734 repeats auto-restart time counting until the
junction temperature is lowered less than 140°C.
Different from Short/Open-LED protection and SRSP,
startup sequence doesn’t appear every 4 seconds of
auto-restart delay time because the temperature is
detected by monitoring internally, not by checking
external pin information. Normal startup sequence is
started again when the junction temperature is out of the
hysteresis temperature (140°C).
© 2014 Fairchild Semiconductor Corporation
FL7734 • Rev. 1.0
11
www.fairchildsemi.com
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