TL5001_15 Datasheet, PDF (8/13 Pages) Unisonic Technologies – PULSE-WIDTH-MODULATION CONTROL CIRCUIT

TL5001
 DETAILED DESCRIPTION(Cont.)
LINEAR INTEGRATED CIRCUIT
SHORT-CIRCUIT PROTECTION (SCP)
The UTC TL5001 includes short-circuit protection (see Fig. 3), which turns the power switch off to prevent damage
when the converter output is shorted. When activated, The SCP prevents the switch from being turned on until the
internal latching circuit is reset. The circuit is reset by reducing the input voltage until UVLO becomes active or until
the SCP terminal is pulled to ground externally.
When a short circuit occurs, the error-amplifier output at COMP rises to increase the power-switch duty cycle in an
attempt to maintain the output voltage. SCP comparator 1 starts an RC timing circuit when COMP exceeds 1.5V.If
the short is removed and the error-amplifier output drops below 1.5V before time out, normal converter operation
continues. If the fault is still present at the end of the time-out period, the time sets the latching circuit and turns off
the UTC TL5001 output transistor.
2.5V
RSCP 185kΩ
CSCP _SCP
5 _{From Error}
Amp
1.5V
scp
Comparator 1
Q2
12kΩ
scp
Comparator 2
Q1
VREF=1V
To Output
Drive Logic
Fig. 3 SCP Circuit
The timer operates by charging an external capacitor (C_SCP),connected between the SCP terminal and ground,
towards 2.5V through a 185k resistor (R_SCP).The circuit begins charging from an initial voltage of approximately
185mV and times out when capacitor voltage reaches 1V.The output of SCP comparator 2 then goes high, turns on
Q2,and latches the timer circuit. The expression for setting the SCP time period is derived from the following
equation:
V_SCP =(2.5-0.185)(1-e^-t/)+0.185
Where
=R_SCPC_SCP
The end of the time-out period, t_SCP, occurs when V_SCP=1V. Solving for C_SCP yields:
C_SCP=12.46*t_SCP
Where
t is in seconds, C in µF
t_SCP must be much longer (generally 10 to 15 times) than the converter start-up period or the converter will not
start.
OUTPUT TRANSISTOR
The output of the UTC TL5001 is an open-collector transistor with a maximum collector current rating of 21mA and
a voltage rating of 51V.The output is turned on under the following conditions: the oscillator triangle wave is lower
than both the DTC voltage and the error-amplifier output voltage, the UVLO circuit is inactive, and the short-circuit
protection circuit is inactive.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
8 of 13
QW-R103-016.E