RT8072
11
DS8072-02 August 2014 www.richtek.com
©
Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Application Information
The basic RT8072 application circuit is shown in Typical
Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating frequency
followed by C
IN
and C
OUT
.
Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation :
FB1
OUT REF
FB2
R
VV1
R
RT8072
GND
FB
R
FB1
V
OUT
R
FB2
Figure 1. Setting the Output Voltage
Soft-Start (SS)
An internal current source charges an external capacitor
to build the soft-start ramp voltage (V
SS
). The V
FB
voltage
will track the V
SS
during soft-start interval. The chip will
use internal soft-start if the SS pin is floating. The nominal
internal soft-start time is 800μs.
With external soft-start, the typical soft-start time can be
calculated as following equation :
t
SS
(ms) = 0.1 x C
SS
(nF)
For example, if C
SS
= 10nF, the soft-start time is 1ms.
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequency improves efficiency by
reducing internal gate charge and switching losses but
requires larger inductance and/or capacitance to maintain
low output ripple voltage.
The operating frequency of the RT8072 is determined by
an external resistor that is connected between the SHDN/
RT pin and GND. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator. The RT resistor value
can be determined by examining the frequency vs. RT
curve. Although frequency as high as 2MHz is possible,
the minimum on-time of the RT8072 imposes a minimum
limit on the operating duty cycle. The minimum on-time
is typically 80ns. Therefore, the minimum duty cycle is
equal to 100 x 80ns x f (Hz).
Chip Enable Operation
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shut down the device. During shutdown
mode, the RT8072 quiescent current drops to lower than
2μA. Driving the EN pin high (>1.5V, <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 3).
where V
REF
equals to 0.8V (typical)
The resistive divider allows the FB pin to sense a fraction
of the output voltage as shown in Figure 1.
Ω
Figure 2
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 60 80 100 120 140 160 180 200
RT (k )
Switching Frequency (MHz) 1
Figure 3. Enable Timing Control
RT8072
EN
GND
V
IN
R
EN
C
EN
EN
