RT7262E
Application Information
The IC is a synchronous high voltage step-down converter
that can support the input voltage range from 4.5V to 21V
and the output current can be up to 2A.
Soft-Start for WDFN-14L Package
The RT7262EZQW (WDFN-14L package) contains an
external soft-start clamp that gradually raises the output
voltage. The soft-start timing is programmed by the
external capacitor between SS pin and GND. The chip
provides an internal 10μAcharge current for the external
capacitor. If 47nF capacitor is used to set the soft-start,
the period will be 4.7ms (typ.).
Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation :
R1
R2
⎛
⎝
⎞
⎟
⎠
VOUT = VFB 1+
⎜
where VFB is the feedback reference voltage 0.808V
(typical).
Soft-Start for SOP-8 (Exposed Pad) Package
The RT7262EZSP (SOP-8 (Exposed Pad) package)
contains an internal soft-start function to prevent large
inrush current and output voltage overshoot when the
converter starts up. Soft-start automatically begins once
the chip is enabled. During soft-start, the internal soft-
start capacitor becomes charged and generates a linear
ramping up voltage across the capacitor. This voltage
clamps the voltage at the internal reference, causing the
duty pulse width to increase slowly and in turn reduce the
output surge current. The typical soft-start time for this
IC is set at 2ms.
The resistive divider allows the FB pin to sense a fraction
of the output voltage as shown in Figure 1.
V
OUT
R1
FB
RT7262E
GND
R2
Figure 1. Output Voltage Setting
External Bootstrap Diode
Under Voltage Lockout Threshold
The IC includes an input Under Voltage Lockout Protection
(UVLO). If the input voltage exceeds the UVLO rising
threshold voltage (4.2V), the converter resets and prepares
the PWM for operation. If the input voltage falls below the
UVLO falling threshold voltage (3.8V) during normal
operation, the device stops switching. The UVLO rising
and falling threshold voltage includes a hysteresis to
prevent noise caused reset.
Connect a 100nF low ESR ceramic capacitor between
the BOOT pin and SW pin as shown in Figure 2. This
capacitor provides the gate driver voltage for the high side
MOSFET. It is recommended to add an external bootstrap
diode between an external 5V and BOOT pin for efficiency
improvement when input voltage is lower than 5.5V or duty
ratio is higher than 65% .The bootstrap diode can be a
low cost one such as IN4148 or BAT54. The external 5V
can be a 5V fixed input from system or a 5V output of the
IC.Note that the external boot voltage must be lower than
Chip Enable Operation
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shutdown the device. During shutdown
mode, the RT7262E quiescent current drops to lower than
1μA. Driving the ENpin high (2V < EN< 5.5V) will turn on
the device again. For external timing control, the EN pin
can also be externally pulled high by adding a REN resistor
and CEN capacitor from the VIN pin (see Figure 3).
5.5V.
5V
BOOT
100nF
RT7262E
SW
Figure 2. External Bootstrap Diode
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7262E-00 November 2012
www.richtek.com
11
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