RT1720

Copyright © 2014 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

DS1720-00

November

2014

www.richtek.com

13

If the OV event is shorter than the TMR charge timing

then examine the MOSFET’s safe operating area (SOA)

determine if the over-voltage event will cause MOSFET

MOSFET’s SOA limits.

If the OV event lasts more than one TMR cycle then the

MOSFET will turn on and off, dissipating power each

time it is on and linear regulating and cooling down

when it is off. In this case, use one of the longer-timed

areas of the SOA graph but adjust the drain current

value by the 12% duty cycle of the MOSFET on periods

to determine if the MOSFET will work. For thermal

management, the MOSFET dissipation during long

over-voltage events is :





MOSFET(OV)

IN

OUT

LOAD(VOUT)

PD

= DC

V

- V

I



where DC is the duty cycle of linear regulation, typically

about 12%.

During an over-current (OC) event the MOSFET will

regulate the output current delivered to the load and

the output voltage will collapse to whatever voltage is

needed to sustain the OC threshold current. According

to the timing determined by the capacitor connected at

the TMR pin, the circuit will turn the load on and off

periodically until the over-current event ends. While

regulating the load current, the MOSFET will dissipate

power and heat up. Unlike an OV event, the output

voltage and the MOSFET’s drain-source voltage may

not be easily predicted. If the output is shorted the

voltage may collapse nearly to zero, placing the entire

input voltage across the MOSFET. Further, this type of

event is likely to continue for long periods. If the output

voltage during the OC event is not easily determined,

For the rare OC event that is short compared to the

TMR timing, examine the MOSFET’s safe operating

current, to determine if the over-current event will

cause MOSFET damage.

If the OC event lasts more than one TMR cycle then the

MOSFET will turn on and off, dissipating power each

time it is on and cooling down when it is off. In this case,

use one of the longer timed areas of the SOA graph

value by the 12% duty cycle of the MOSFET on periods

to determine if the MOSFET will work. For thermal

management, the MOSFET dissipation during long

over-current events is :





MOSFET(OV)

IN

OUT

OC_THRESHOLD

PD

= DC

V

- V

I



where DC is the duty cycle of current regulation,

typically about 12%.

Parallel MOSFETs

Select a single MOSFET for most applications. If the

necessary voltage rating, multiple devices may be used

in parallel. Parallel devices can decrease the voltage

drop in normal operation and reduce dissipation.

However, SOA requirements must generally be met by

a single device.

In OV and OC conditions, GATE will decrease until the

programmed output voltage or current is maintained. In

that state, the MOSFET with the lowest threshold will

carry more current than other parallel MOSFETs with

higher thresho

lds, perhaps dramatically more. It’s

generally best to assume that one device will be

subjected to the entire SOA stress.

Shutdown

The RT1720 enters a low current (7

A typical)

shutdown mode when the voltage at the

SHDN

̅̅̅̅̅̅̅̅̅ pin

goes below its 0.5V logic-low level. In shutdown all

functions

are

turned

off, including

the

auxiliary

amplifier.

For automatic start-

up, it’s recommended to connect

SHDN pin to

(1) A voltage between 3V and 45V through a 100k

resistor.

(2) A voltage higher than 45V through a resistor with

minimum value of the following formula,

No external voltage clamp is needed since the pin

clamps the input voltage. If external board leakage is

kept below 1

A, the pin can be left open and an

internal current source will pull the pin voltage to about

2.5V.

SHDN

̅̅̅̅̅̅̅̅̅ may also be driven by a logic output to

turn the IC on and off.