STEP-UP DC/DC CONTROLLER

2007-6-6

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FSP3601

Oscillator

The oscillator frequency can be set between 20 KHz and 500 KHz by connecting a resistor and a capacitor at OSC

pin of FSP3601 to ground. The oscillator frequency can be determined by using the graph shown in figure 9.

The oscillator output is a sawtooth wave with a minimum value of approximately 0.1V and a maximum value of

approximately 0.6V. The PWM comparator compares the oscillator voltage with error amplifier output voltage; internal

DTC voltage (typ.0.6V) and soft start setting voltage. When the sawtooth wave is lower than the above three output

voltage, the output of FSP3601 is high (Turn on NPN transistor or NMOS).

Under voltage lockout (UVLO)

The under voltage lockout circuits turn the output off whenever the supply voltage drop too low (approximately 0.9V

at 25 ℃) for proper operation .A hysterics voltage of 200 mV eliminates false triggering on noise and chattering.

Soft Start / Short-circuit protection (S.S. / S.C.P.)

The soft start is function after power on, and the interval of the soft start time is determined by capacitor connected to

SCP pin (pin 2). When soft start function finished, the internal soft start voltage is setting high, but external SCP pin is

setting low in order to change to short circuit detection / protection function.

The time of soft start is following expression:

The short circuit protection is functional due to a heavy loading drop and output of error amplifier (COMP pin) is

disable (internal pull-low ) and the capacitor is discharged to low .

The time of short circuit protection is following expression:

Output transistor

The FSP3601 has a totem-pole transistor with a 40 mA source/sink current rating to drive an external NPN transistor

or NMOS transistor directly. The driving current capability depends on a resistor R that is connected to BR/CTL pin

(pin4) of FSP3601. (see fig.13)

Figure13. Output transistor driving control circuit

BR/CTL pin can also use to control the output of FSP3601 for disable or enable function of system.

Control Pin

Q1

BR/CTL Pin

Output Transistor Function

Mode

Low

Off

Open

Disable

Stand-by

High

On

Bias Current

Enable

Operation

BR/CTL

R

Control

Q1

FSP3601

4