RT8223P datasheet(19/23 Pages) RICHTEK

19 / 23 page

RT8223P

DS8223P-01

June 2011

www.richtek.com

The average drive current is calculated by the gate charge

at VGS = 5V times the switching frequency. The

instantaneous drive current is supplied by the flying

capacitor between the BOOTx and PHASEx pins. A dead

time to prevent shoot through is internally generated

between high side MOSFET off to the low side MOSFET

on, and the low side MOSFET off to the high side MOSFET

on.

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

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

that drives LGATEX low is robust, with a 1.5

Ω typical on-

resistance. A 5V bias voltage is delivered from the VREG5

supply. The instantaneous drive current is supplied by an

input capacitor connected between VREG5 and GND.

For high current applications, some combinations of high

and low side MOSFETs might be encountered that will

cause excessive gate-drain coupling, which can lead to

efficiency killing, EMI-producing shoot-through currents.

This can be 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 (Figure 3).

Figure 3. Increasing the UGATEx Rise Time

Soft-Start

The RT8223P 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 2 ms. A unique PWM duty limit control

that prevents output over voltage during soft-start period

is designed specifically for FBx floating.

UVLO Protection

The RT8223P features VREG5 under voltage lockout

protection (UVLO). When the VREG5 voltage is lower than

3.9V (typ.) and the VREG3 voltage is lower than 2.5V

(typ.), both switch power supplies are shut off. This is

non-latch protection.

Power Good Output (PGOOD)

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

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

standby, and shutdown. It is released when both output

voltages are above 91% of the nominal regulation point.

The PGOOD goes low if either output turns off or is 15%

below its nominal regulator point.

Output Over Voltage Protection (OVP)

The output voltage can be continuously monitored for over

voltage. If the output voltage exceeds 12% 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 input voltage downward.

The RT8223P is latched once OVP is triggered and can

only be released by toggling EN, ENTRIPx or cycling VIN.

There is a 5

μs delay built into the over voltage protection

circuit to prevent false alarm.

Note that the LGATEx latching high causes the output

voltage to dip slightly negative when energy has been

previously stored 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 short in the

high side switch, completely turning on the low side

MOSFET can create an electrical short between the

battery and GND, which will blow the fuse and disconnect

the battery from the output.

Output Under Voltage Protection (UVP)

The output voltage can be continuously monitored for under

voltage protection. If the output is less than 52% of its set

voltage threshold, under voltage protection will be triggered,

and then both UGATEx and LGATEx gate drivers will be

forced low. The UVP will be ignored for at least 5ms (typ.)

after start-up or a rising edge on ENTRIPx. Toggle

BOOTx

UGATEx

PHASEx

RBOOT

VIN

RT8223P Datasheet(PDF) 19 Page - Richtek Technology Corporation

RT8223P Datasheet(HTML) 19 Page - Richtek Technology Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL