RT4723P
Application Information
The RT4723P is a highly integrated Boost, LDO and
inverting charge pump to generate positive and
negative output voltages for AMLOED bias. It can
support input voltage range from 2.5V to 4.6V and the
output current up to 30mA. The VOP positive output
voltage is generated from the LDO supplied from a
synchronous Boost converter, and VOP is set at a
typical value of 4.6V. The Boost converter output also
drives an inverting charge pump controller to generate
VON negative output voltage which is set at a typical
value of 2.4V. The negative output voltage can be
programmed through the dedicated pin which
implements single wire protocol and the available
voltage range is from 0.6V to 2.4V with 100mV per
step.
The inductance can eventually be determined
according to the following equation :
η V 2 VOUT V
IN
IN
L
0.4 V
2 IOUT(MAX)fOSC
OUT
where fOSC is the switching frequency. For better
system performance, a shielded inductor is preferred to
avoid EMI problems.
Boost Output Capacitor Selection
The output ripple voltage is an important index for
estimating IC performance. This portion consists of two
parts. One is the product of ripple current with the ESR
of the output capacitor, while the other part is formed
by the charging and discharging process of the output
capacitor. As shown in Figure 1, VOUT1 can be
evaluated based on the ideal energy equalization.
According to the definition of Q, the VOUT1 value can
be calculated as the following equation :
Input Capacitor Selection
Input ceramic capacitor with 4.7F capacitance is
suggested for applications. For better voltage filtering,
select ceramic capacitors with low ESR, X5R and X7R
types are suitable because of their wider voltage and
temperature ranges.
1
fSOC
Q = IOUT D
= COUT VOUT1
IOUT D
fSOC COUT
Boost Inductor Selection
VOUT1
=
The inductance depends on the maximum input current.
As a general rule, the inductor ripple current range is
20% to 40% of the maximum input current. If 40% is
selected as an example, the inductor ripple current can
be calculated according to the following equations :
where fOSC is the switching frequency and D is the duty
cycle.
Finally, taking ESR into consideration, the overall
output ripple voltage can be determined by the
following equation :
VOUT IOUT(MAX)
I
=
IN(MAX)
I
D
OUT
V
IN
V
= V
+ V
= V
+
OUT
ESR
OUT1
ESR
f
C
OSC
OUT
IL = 0.4I
IN(MAX)
where VESR = ICrms x RCESR
where η is the efficiency of the VOP Boost converter,
IIN(MAX) is the maximum input current, and IL is the
inductor ripple current. The input peak current can then
be obtained by adding the maximum input current with
half of the inductor ripple current as shown in the
following equation :
The output capacitor, COUT, should be selected
accordingly.
IPEAK = 1.2 x IIN(MAX)
Note that the saturated current of the inductor must be
greater than IPEAK
.
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS4723P-00 December 2016
www.richtek.com
11
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