RT6551A/B
Operation
The RT6551A/B is a constant on-time synchronous step-
down controller. In normal operation, the high-side
N-MOSFET is turned on when the output voltage is lower
than VREF, and is turned off after the internal one-shot
timer expires. While the high-side N-MOSFET is turned
off, the low-side N-MOSFET is turned on to conduct the
inductor current until next cycle begins.
Soft-Start (SS)
For internal soft-start function, an internal current source
charges an internal capacitor to build the soft-start ramp
voltage. The output voltage will track the internal ramp
voltage during soft-start interval.
PGOOD
The power good output is an open-drain architecture. When
the soft-start is finished, the PGOOD open-drain output
will be high impedance.
Current Limit
The current limit circuit employs a unique
“valley”
current
sensing algorithm. If the magnitude of the current sense
signal at PHASE is above the current limit threshold, the
PWM is not allowed to initiate a new cycle. The current
limit threshold can be set with an external voltage setting
resistor on the CS pin.
Over-Voltage Protection (OVP) & Under-Voltage
Protection (UVP)
The output voltage is continuously monitored for over-
voltage and under-voltage protection. When the output
voltage exceeds its set voltage threshold( 115% of V
OUT
),
UGATE goes low and LGATE is forced high. When the
feedback voltage is less than 0.45V, under-voltage
protection is triggered and then both UGATE and LGATE
gate drivers are forced low. The controller is latched until
VDD is re-supplied and exceeds the POR rising threshold
voltage or S5 is reset.
VTT Linear Regulator and VTTREF
This VTT linear regulator employs ultimate fast response
feedback loop so that small ceramic capacitors are enough
for keeping track of VTTREF within 40mV at all conditions,
including fast load transient. The VTTREF block consists
of on-chip 1/2 divider, LPF and buffer. This regulator also
has sink and source capability up to 10mA. Bypass
VTTREF to GND with a 33nF ceramic capacitor for stable
operation.
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6551A/B-00 May 2015
www.richtek.com
5
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
