RT7235/36/39/40
Function Block Diagram
VCC*
VREG5
BOOT
POR &
Reg
EN
Min.
VREG5
Off-Time
VIN
SW
VBIAS
OC
V
REF
Control
Driver
ZC
UV & OV
PGND
GND*
SW
V
REG5
Ripple
Gen.
2µA
+
-
-
SS
Comparator
-
PGOOD*
FB
V
IN
FB
Comparator
+
0.9 x V
REF
FB
On-Time
VS*
* : VCC pin for TSSOP-14 (Exposed Pad) only.
GND pin for TSSOP-14 (Exposed Pad) only.
VS pin for TSSOP-14 (Exposed Pad) only.
PGOOD pin for TSSOP-14 (Exposed Pad) and WDFN-10L 3x3 only.
Operation
The RT7235/36/39/40 is a synchronous step-down
converter with advanced constant on-time control mode.
Using the ACOTTM control mode can reduce the output
capacitance and provide fast transient response. It can
minimize the component size without additional external
compensation network.
Current Protection
The inductor current is monitored via the internal switches
in cycle-by-cycle. Once the output voltage drops under
UV threshold, the device will enter latch mode for TSSOP-
14 (Exposed Pad), and hiccup mode for WDFN-10L 3x3.
UVLO Protection
Power Good
To protect the chip from operating at insufficient supply
voltage, the UVLO is needed. When the input voltage of
VCC is lower than the UVLO falling threshold voltage, the
device will be latch-off.
(for TSSOP-14 (Exposed Pad) and WDFN-10L 3x3 only)
After soft-start is finished, the power good function will be
activated. When the FB is activated, the PGOOD will
become an open-drain output. If the FB is below, the
PGOOD pin will be pulled low.
Output Discharge Control
(for TSSOP-14 (Exposed Pad) only)
Internal Regulator
When EN pin is low, the RT7235/36 will discharge the
output with an internal 50Ω MOSFET connected between
VOUT to GND pin.
The regulator provides 5V power to supply the internal
control circuit. Connecting a 1μF ceramic capacitor for
decoupling and stability is required.
Thermal Shutdown
Soft-Start
When the junction temperature exceeds the OTP
threshold value, the IC will shut down the switching
operation. Once the junction temperature cools down and
is lower than the OTP lower threshold, the converter will
automatically resume switching
In order to prevent the converter output voltage from
overshooting during the startup period, the soft-start
function is necessary. The soft-start time is adjustable
and can be set by an external capacitor.
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS7235/36/39/40-00 March 2013
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
