RT2702
Application Information
The RT2702 is a high-efficiency and high-current
synchronous buck controller designed to drive a DrMOS
module. ACOT (Constant-On-Time) PWM controller and
PWM signal forDrMOS are integrated so that the external
circuit is easily designed and the component count is
reduced. The RT2702 adopts FCOT control technology
that responds instantly to input voltage change. Besides,
the RT2702 includes a 5V (VDRV) linear regulator, which
can provide 5V voltage and at least 100mAcurrent loading.
limit threshold and the minimum off-time one shot has
timed out.
V
FB
V
PEAK
V
V
FB
VALLEL
V
REF
t
The topology circumvents the poor load transient timing
problems of fixed-frequency current mode PWMs while
avoiding the problems caused by widely varying switching
frequency in conventional constant-on-time and constant-
off-time PWM schemes.
t
ON
Figure 1. Constant On-Time PWM Control
The IC supports two modes transition function with various
operating states, which include audio skipping mode
(ASM) and forced-CCM mode (FCCM). These different
operating eliminates audio-frequency modulation and
states make the system efficiency as high as possible.
On-Time Control
A resistor, RTON, is connected between VIN and TON
generates on-time current forward to TONGen. block. With
different VINvoltage and RTON value, the on-time current
has different value. This on-time current charges an internal
capacitor, CTON, which is about 3.8pF, then the on-time
will be the voltage on CTON from 0V to VOUT. Besides, the
switching frequency, fSW, is roughly determined by RTON
and CTON without a clock generator.
The device can operate at wide input voltage range from
4.5 V to 19V. It is capable of producing an output voltage
as low as 0.6V. Other features include power good
indication, external soft start setting, soft-off discharge
function, OVP, UVP, Current Limit, precise DCR sense,
and enable/disable control.
RTON VOUT 3.8p
tON
VIN 1.17
PWM Operation
And the specific switching frequency fSW is :
The RT2702 integrates a Constant-On-Time PWM
controller, and the controller provides the PWM signal
which relies on the FB voltage comparing with internal
reference voltage as shown in Figure 1. Refer to the
function block diagram of the RT2702, the synchronous
high side MOSFET of DrMOS will be turned on at the
beginning of each cycle. After the internal one shot timer
expires, the MOSFET will be turned off. The pulse width
of this one shot is determined by the converter’s input
voltage and the output voltage to keep the frequency
constant in a specific input voltage range. Another one
shot sets a minimum off-time (275ns typ.). The on-time
one shot will be triggered if the PWM comparator output
is high, the low side switch current is below the current
VOUT
fSW
V
IN tON
The recommended operating frequency is between 200kHz
and 1.2MHz.
Mode Selection
The RT2702 operates under audio skipping mode (ASM)
or force-CCM (FCCM) according to MODE pin voltage
setting. If MODE voltage is pulled below 0.4V, controller
turns on ASM function. If MODE voltage is higher than
1.2V, controller enters forced-CCM.
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS2702-02 May 2019
www.richtek.com
12
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