RT2702
Operation
The RT2702 includes a constant on-time synchronous
step-down controller and a linear regulator.
Buck Controller
In normal operation, the high-side N-MOSFET of DrMOS
is turned on when the V
FB
is lower than V
REF
, and is turned
off after the internal one-shot timer expires. While the
high-side N-MOSFET of DrMOS is turned off, the low-side
N-MOSFET of that is turned on to conduct the inductor
current until next cycle begins.
Soft-Start
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. For external soft-start
function, an additional capacitor, connected between SS
pin and GND, will be charged by a current source and
determines the soft-start time. The RT2702 will track the
slower one soft-start ramp during soft-start process.
POK
The power ok is the output of open-drain buffer. When the
soft-start process is finished, this pin becomes high
impedance.
Current Limit
The current limit circuit employs a unique
“valley”
current
sensing algorithm. If V
CSP
is higher than V
CSN
, the PWM
is not allowed to initiate a new cycle. Thus, the current to
the load exceeds the average output inductor current,
eventually, output drops lower than the under-voltage
protection threshold, inducing IC shutdown.
Over-Voltage Protection (OVP) & Under-Voltage
Protection (UVP)
The output voltage is continuously monitored for over-
voltage and under-voltage conditions. When the voltage
of FB pin exceeds over-voltage threshold (112.5% of V
REF
),
the PWM signal goes low. When it is less than 87.5% of
V
REF
, under-voltage protection is triggered and then the
DISB signal will be pulled low to disable DrMOS. The
controller begins hiccup procedure and restart up after
20ms.
VDRV
Internal LDO turns on When the V
IN
and EN voltage exceed
the POR rising threshold, respectively. Provide 5V supply
voltage at VDRV pin.
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS2702-02 May 2019
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
