RT8206A/B
MOSFET Gate Driver (UGATEx, LGATEx)
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.
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.Adead 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.
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.
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 0.6Ω 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 andGND.
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.)
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).
V
IN
Output Under-Voltage Protection (UVP)
10
BOOTx
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.
UGATEx
PHASEx
Figure 5. Reducing the UGATEx Rise Time
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.
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
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
DS8206A/B-03 December 2009
www.richtek.com
21
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
