RT7231/32/33/34
Output Voltage Setting
highest efficiency operation. However, it requires a large
inductor to achieve this goal. For the ripple current
selection, the value of ΔIL = 0.2(IMAX) will be a reasonable
starting point. The largest ripple current occurs at the
highest VIN. To guarantee that the ripple current stays
below the specified maximum, the inductor value should
be chosen according to the following equation :
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 4.
V
OUT
R1
FB
RT7231/32/33/34
R2
V
f I
V
OUT
V
IN(MAX)
OUT
L =
1
L(MAX)
GND
Figure 4. Output Voltage Setting
Input and Output Capacitors Selection
The input capacitance, CIN, is needed to filter the
trapezoidal current at the source of the high side MOSFET.
Alow ESR input capacitor with larger ripple current rating
should be used for the maximum RMS current. The RMS
current is given by :
The output voltage is set by an external resistive divider
according to the following equation. It is recommended to
use 1% tolerance or better divider resistors.
R1
VOUT = 0.765(1
)
R2
V
V
V
IN
V
OUT
OUT
I
= I
1
RMS
OUT(MAX)
Under Voltage Lockout Protection
IN
The RT7231/32/33/34 has Under Voltage Lockout
Protection (UVLO) that monitors the voltage of PVCC pin.
When the VPVCC voltage is lower than UVLO threshold
voltage, the RT7231/32/33/34 will be turned off in this state.
This is non-latch protection.
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT / 2. This simple worst-case condition is
commonly used for design because even significant
deviations do not offer much relief.
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design. For the
input capacitor, two 10μF and 0.1μF low ESR ceramic
capacitors are recommended.
Over Temperature Protection
The RT7231/32/33/34 equips an Over Temperature
Protection (OTP) circuitry to prevent overheating due to
excessive power dissipation. The OTP will shut down
switching operation when junction temperature exceeds
150°C. Once the junction temperature cools down by
approximately 20°C the main converter will resume
operation. To keep operating at maximum, the junction
temperature should be prevented from rising above 150°C.
The selection of COUT is determined by the required ESR
to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for COUT selection to ensure that the control loop is stable.
The output ripple, ΔVOUT , is determined by :
1
Inductor Selection
VOUT I ESR
L
8fCOUT
The inductor value and operating frequency determine the
ripple current according to a specific input and an output
voltage. The ripple current ΔIL increases with higher VIN
and decreases with higher inductance.
The output ripple will be highest at the maximum input
voltage since ΔIL increases with input voltage. Multiple
capacitors placed in parallel may need to meet the ESR
and RMS current handling requirements.
V
f L
VOUT
V
IN
OUT
1
IL =
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
Copyright 2016 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7231/32/33/34-09 October 2016
www.richtek.com
13
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