OPERATION DESCRIPTION

The PS1313 is a high efficiency, fixed frequency, peak

current mode boost regulator. It has the architecture to

regulate the voltage at the feedback pin, so that a

regulated fixed current is achieved to drive white LEDs.

The power MOS is turned ON through the control

circuitry, at the start of each oscillator cycle and thus the

charging phase is initiated. The error amplifier, consisting

of a voltage comparator and current sense amplifier, is

basically, a PWM comparator. The voltage comparator

amplifies the difference between the reference and feed

back voltage. When the output of the current sense

amplifier, reaches the output of the voltage comparator,

the POWER MOS is turned OFF and thus the charging

phase is terminated. To prevent sub-harmonic oscillations

at duty cycles greater than 50 percent, a stabilizing ramp

is added to the current sense signal. In this way the peak

current level keeps the output in regulation. The PS1313

has internal soft start mechanism, to limit the inrush

current at startup and to limit the amount of overshoot

on the output, also. An internal blanking time is provided

during start up, to prevent the start of switching before

all the circuitry become ready for operation. The current

limit is increased by a fourth every 60µs giving a total

soft start time of 240µs.

Setting the LED current :

The LED current can be set, according to the

requirement, by feedback to ground resistor, R1.

The current through the LEDs is selected by the

following equation:

Current Limit :

The PS1313 includes a current limiter. It monitors

the peak current through the inductor and controls

gate of the power device.

Enable Input :

The PS1313 features an active-high Enable input

(EN) pin that allows on/off control of the regulator.

The PS1313 bias current reduces to less than

microampere when it is shutdown. The output

remains at a schottky forward voltage lower than

the SW pin, if a schottky diode is connected

between SW and OV pin, at shut. The Enable input

is TTL/CMOS compatible.

PS1313

Under Voltage Lockout:

When the input supply goes too low (below 2.15V)

the PS1313 produces an internal UVLO (under

voltage lockout) signal that shuts down the chip.

This mechanism protects the chip from producing

false logic due to low input supply.

Thermal Overload Protection:

July 2007

www.picsemi.com

© 2007 Power IC. All Rights Reserved.

6

Thermal-overload protection limits total power

dissipation in the PS1313. When the junction

sensor signals the shutdown logic and turning off

most of the internal circuitry. The thermal sensor

turns internal circuitry on again after the IC’s

junction temperature drops by 20

°C. The regulator

then starts functioning in the required mode based

on the supply voltage.

Thermal-Overload protection is designed to protect

the PS1313 in the event of a fault condition. For

continual operation, do not exceed the absolute

maximum junction temperature rating of Tj =

+150

°C.

Open Load Protection:

Open load protection in PS1313, protects the chip

from destruction, due to excessive high voltage.

When, in any case, one or more LEDs in the LED

string fails, the feedback pin is pulled down to zero.

As a result, the chip runs at maximum duty cycle,

boosting the output voltage higher and higher. The

open load protection mechanism, checks this

condition, if OV pin is tied to the top of the LED

string. If the output voltage exceeds the OV

threshold (18V) and persists there for a certain

span of time (15µS), the switching stops, allowing

the output to discharge. The open load protection

mechanism includes a hysteretic comparator. The

switching is enabled again, when the output voltage

has fallen to a certain level. Because the chip

doesn’t turn OFF fully, no power recycling is

necessary, when this condition takes place. But it’s

highly recommended, not to use the chip in that

mode of operation for longer span of time.