RT7266
Application Information
The RT7266 is a synchronous high voltage buck converter
that can support the input voltage range from 4.5V to 18V
and the output current can be up to 3A. It operates using
adaptive on-time ACOT
TM
mode control and provides a very
fast transient response with few external compensation
components. The RT7266 allows low external component
count configuration with both low ESR and ceramic output
capacitors.
PWM Operation
It is suitable for low external component count
configuration with appropriate amount of Equivalent Series
Resistance (ESR) capacitor(s) at the output. The output
ripple valley voltage is monitored at a feedback point
voltage. The synchronous high side MOSFET is turned
on at the beginning of each cycle. After the internal one
shot timer expires, the MOSFET is turned off. The pulse
width of this one shot is determined by the converter's
input and output voltages to keep the frequency fairly
constant over the entire input voltage range.
Adaptive On-Time Control
The RT7266 has a unique circuit to ensure the switching
frequency on 700kHz over full input voltage range and full
loading range. This circuit sets the on-time one-shot timer
by monitoring the input voltage and SW signal. The
switching frequency will keep constant if the duty ratio is
V
OUT
/V
IN
.
Duty Ratio = V
OUT
/V
IN
= t
ON
/ T
Chip Enable
* : Optional
C5
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 RT7266 quiescent current drops to lower than
10μA. Driving the EN pin high (>2V, <5.5V) will turn on
the device again. For external timing control, the EN pin
can also be externally pulled high by adding a R
EN
* resistor
and C
EN
* capacitor from the VIN pin (see Figure 1).
V
IN
4.5V to 18V
R
EN
*
Chip Enable
C
EN
*
4 SS
5,
9 (Exposed Pad)
GND
FB 2
PVCC
3
C4
R2
1 EN
8
C1
7
C6
SW 6
L1
V
OUT
VIN
BOOT
RT7266
R1
C7
Figure 1. External Timing Control
An external MOSFET can be added to implement digital
control on the EN pin when no system voltage above 2V
is available, as shown in Figure 2. In this case, a 100kΩ
pull-up resistor, R
EN
, is connected between V
IN
and the
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
V
IN
R
EN
100k
Q1
4 SS
C5
5,
9 (Exposed Pad) GND
FB 2
PVCC 3
C4
R2
C1
1 EN
8
VIN
BOOT
RT7266
SW 6
7
C6
V
OUT
L1
R1
C7
For Fixed T, Ton is proportional to V
OUT
/V
IN
.
Soft-Start
The RT7266 contains an external soft-start clamp that
gradually raises the output voltage. The soft-start timing
can be programmed by the external capacitor between
SS pin and GND. The chip provides a 2μA charge current
for the external capacitor. If a 3.9nF capacitor is used,
the soft-start will be 2ms (typ.). The available capacitance
range is from 2.7nF to 220nF.
t
SS
(ms) =
C5 (nF)
×
1.065
I
SS
(
μ
A)
Figure 2. Logic Control with Low Voltage
Copyright
©
2012 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7266-02
September 2012
www.richtek.com
9
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
