RT8249 datasheet(18/23 Pages) RICHTEK

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RT8249A/B/C

DS8249A/B/C-02

June 2014

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Figure 4. Increasing the UGATEx Rise Time

Soft-Start

The RT8249A/B/C provides an internal soft-start function

to prevent large inrush current and output voltage overshoot

when the converter starts up. The soft-start (SS)

automatically begins once the chip is enabled. During soft-

start, it clamps the ramping of internal reference voltage

which is compared with FBx signal. The typical soft-start

duration is 0.9ms. An unique PWM duty limit control that

prevents output over-voltage during soft-start period is

designed specifically for FBx floating.

UVLO Protection

The RT8249A/B/C has LDO5 under-voltage lock out

protection (UVLO). When the LDO5 voltage is lower than

3.9V (typ.) and the LDO3 voltage is lower than 2.5V (typ.),

both switch power supplies are shut off. This is a non-

latch protection.

BOOTx

RBOOT

UGATEx

VIN

PHASEx

Power Good Output (PGOOD)

PGOOD is an open-drain output and requires a pull-up

resistor. PGOOD is actively held low in soft-start, standby,

and shutdown. For RT8249A/B/C, PGOOD is released

when both output voltages are above 88% of nominal

regulation point. The PGOOD signal goes low if either

output turns off or is 20% below or 13% over its nominal

regulation point.

Output Over-Voltage Protection (OVP)

The output voltage can be continuously monitored for over-

voltage condition. If the output voltage exceeds 13% of

its set voltage threshold, the over-voltage protection is

triggered and the LGATEx low-side gate drivers are forced

high. This activates the low-side MOSFET switch, which

rapidly discharges the output capacitor and pulls the output

voltage downward.

The RT8249A/B/C is latched once OVP is triggered and

can only be released by either toggling ENx or cycling

VIN. There is a 1

μs delay built into the over-voltage

protection circuit to prevent false transition.

Note that latching LGATEx high will cause the output

voltage to dip slightly negative due to previously stored

energy in the LC tank circuit. For loads that cannot tolerate

a negative voltage, place a power Schottky diode across

the output to act as a reverse polarity clamp.

If the over-voltage condition is caused by a shorted in

high-side switch, turning the low-side MOSFET on 100%

will create an electrical shorted circuit between the battery

and GND to blow the fuse and disconnecting the battery

from the output.

Output Under-Voltage Protection (UVP)

The output voltage can be continuously monitored for under-

voltage condition. If the output is less than 52% (typ.) of

its set voltage threshold, the under-voltage protection will

be triggered and then both UGATEx and LGATEx gate

drivers will be forced low. The UVP is ignored for at least

1.3ms (typ.) after a start-up or a rising edge on ENx. Toggle

ENx or cycle VIN to reset the UVP fault latch and restart

the controller.

shoot through is internally generated from high-side

MOSFET off to low-side MOSFET on and low-side

MOSFET off to high-side MOSFET on.

The low-side driver is designed to drive high current low

RDS(ON) N-MOSFET(s). The internal pull down transistor

that drives LGATEx low is robust, with a 1

Ω typical on-

resistance. A 5V bias voltage is delivered from the LDO5

supply. The instantaneous drive current is supplied by an

input capacitor connected between LDO5 and GND.

For high current applications, some combinations of high

and low-side MOSFETs may cause excessive gate drain

coupling, which leads to efficiency killing, EMI producing,

and shoot through currents. This is often remedied by

adding a resistor in series with BOOTx, which increases

the turn-on time of the high-side MOSFET without

degrading the turn-off time. See Figure 4.

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