RT4722C
Application Information
The RT4722C is a triple channels DC-DC converter,
which integrates dual step up converter and an
inverting converter to provide the positive and negative
output voltage required by AMOLED. Both positive
(VO1) and negative (VO2) voltage can be programmed
by external MCU through single wire (SWIRE pin) and
another step up converter is from 5.8V to 7.9V output
voltage by SWIRE pin. The RT4722C protection
function includes Over-Temperature Protection (OTP),
Over-Current Protection (OCP) and Short Circuit
Protection (SCP), VO2 and VO3 have Pulse Skipping
Mode (PSM) to provide high efficiency during light load.
If OCP occurs keep more than 1ms IC will shut down
and need reset enable signal to recovery.
Short Circuit Protection (SCP)
The RT4722C has an advanced short circuit protection
mechanism which prevents damage to the device from
unexpected applications. When the output becomes
lower than about 90% shorted to ground, over 1ms the
device enters shutdown mode.
Under-Voltage Lockout (UVLO)
To prevent abnormal operation of the IC in low voltage
condition, an under voltage lockout is included, which
shuts down the device at voltages lower than 2.2V. All
functions will be turned off in this state.
Soft-Start
The RT4722C use an internal soft-start feature to avoid
high inrush currents during step-up.
Input Capacitor Selection
Each channel input ceramic capacitors with 10F
capacitance are suggested for the RT4722C
applications. However, to achieve best performance
with the RT4722C, larger capacitance can be used. For
better voltage filtering, select ceramic capacitors with
low ESR, X5R and X7R types which are suitable
because of their wider voltage and temperature ranges.
It is suggested that system need start loading after soft
start finished and output voltage was ready.
Fast Discharge Function
All outputs voltage use an embedded discharge
function to discharge the remaining output to 0V rapidly,
preventing phenomena such as residual image on the
display during shutdown.
Boost Inductor Selection
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 :
VOUT IOUT(MAX)
Over-Temperature Protection (OTP)
The RT4722C includes an Over-Temperature
Protection (OTP) feature to prevent excessive power
dissipation from overheating the device. The OTP will
shut down switching operation when junction
temperature exceeds 140C. Once the junction
temperature cools down by approximately 15C, the
converter resumes operation.
I
=
IN(MAX)
η V
IN
IL= 0.4I
IN(MAX)
To maintain continuous operation, prevent the
maximum junction temperature from rising above
125C.
where η is the efficiency of the converter, I
is the
IN(MAX)
maximum input current, and I is the inductor ripple
L
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 :
Over-Current Protection (OCP)
The RT4722C includes a current sensing circuitry
which monitors the inductor current during each ON
period after soft start finished. If the current value
becomes greater than the current limit, the switch that
pertains to inductor charging will turn off, forcing the
inductor to leave charging stage and enter discharge
stage.
I
= 1.2×I
IN(MAX)
PEAK
Note that the saturated current of the inductor must be
greater than I
.
PEAK
Copyright © 2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS4722C-00 September 2017
www.richtek.com
13
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