TL5001

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

8 of 13

www.unisonic.com.tw

QW-R103-016,B

DETAILED DESCRIPTION(Cont.)

SHORT-CIRCUIT PROTECTION (SCP)

The UTC TL5001 includes short-circuit protection (see Figure 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

12kO

SCP

From Error

Amp

1.5V

scp

Comparator 1

Q2

Q1

scp

Comparator 2

To Output

Drive Logic

Figure 3.SCP Circuit

5

The timer operates by charging an external capacitor (Cscp),connected between the SCP terminal and ground,

towards 2.5V through a 185k

Ω resistor (Rscp).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:

Vscp =(2.5-0.185)(1-e

-t/

Where

ι=RscpCscp

The end of the time-out period, tscp, occurs when Vscp=1V. Solving for Cscp yields:

Cscp=12.46*tscp

Where

t is in seconds, C in µF

tscp 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.