RT8206A datasheet(21/26 Pages) RICHTEK

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RT8206A/B

DS8206A/B-04 March 2011

www.richtek.com

MOSFET Gate Driver (UGATEx, LGATEx)

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

RDS(ON) N-MOSFET(s). When configured as a floating driver

the instantaneous drive current is supplied by the flying

capacitor between BOOTx and PHASEx pins. A dead time

to prevent shoot through is internally generated between

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 pulldown transistor

that drives LGATEx low is robust, with a 0.6

Ω typical on-

resistance. A 5V bias voltage is typically delivered from

PVCC through LDO supply. The instantaneous drive

current is supplied by an input capacitor connected

between PVCC 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 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 (Figure 5).

Figure 5. Reducing the UGATEx Rise Time

BOOTx

UGATEx

PHASEx

VIN

Soft-Start

A built-in soft-start is used to prevent surge current from

power supply input after ENx is enabled. The typical soft-

start duration is 2ms period. The maximum allowed current

limit is segmented in 5 steps : 20%, 40%, 60%, 80% and

100% during this period. The current limit steps can

eliminate the VOUT folded-back in the soft-start duration.

POR and UVLO

Power-on reset (POR) occurs when VIN rises above

approximately 3.7V (typ.), resetting the fault latches.

PVCC undervoltage-lockout (UVLO) circuitry inhibits

switching by keeping UGATEx and LGATEx low when

PVCC is below 4V. The PWM outputs begin to ramp up

once PVCC exceeds its UVLO threshold and ENx is

enable.

Power-Good Output (PGOODx)

The PGOODx is an open-drain type output. PGOODx is

actively held low in soft-start, standby, and shutdown. It is

released when the VOUTx voltage is above than 92.5% of

the nominal regulation point. The PGOODx goes low if it

is 7.5% below its nominal regulator point.

Output Over voltage Protection (OVP)

The output voltage can be continuously monitored for over

voltage protection. When the output voltage of the VOUTx

is 11% above the set voltage, over voltage protection will

be enabled, if the output exceeds the over voltage

threshold, over voltage fault protection will be 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 reduces the output

voltage. Once an over-voltage fault condition is set, it can

only be reset by toggling ENLDO, ENx, or cycling VIN

(POR.)

Output Under-Voltage Protection (UVP)

The output voltage can be continuously monitored for under

voltage protection. If the output is less than 70% of the

error-amplifier trip voltage, 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

3ms (typ.) after start-up or after a rising edge on ENx.

Toggle ENx or cycle VIN (POR) to clear the under-voltage

fault latch and restart the controller. The UVP only applies

to the BUCK outputs.

Thermal Protection

The RT8206A/B has a thermal shutdown protection

function to prevent it from overheating. Thermal shutdown

occurs when the die temperature exceeds +150

°C. All

internal circuitry will be shut down during thermal shutdown.

The RT8206A/B may trigger thermal shutdown if the LDO

were not supplied from VOUTx, while input voltage is on

VIN and drawing current that is too high from the LDO.

Even if the LDO is supplied from VOUTx, overloading the

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