RT6266
Application Information
1
VOUT
f I
VOUT
L =
Output Voltage Setting
L(MAX)
V
IN(MAX)
The resistive divider allows the FB pin to sense the
output voltage as shown in Figure 1.
The inductor's current rating (caused
a
40C
temperature rising from 25C ambient) should be
greater than the maximum load current and its
saturation current should be greater than the short
circuit peak current limit. Please see Table 2 for the
inductor selection reference.
V
OUT
R1
FB
RT6266
GND
R2
Table 2. Suggested Inductors for Typical
Application Circuit
Component
Supplier
Dimensions
(mm)
Series
Figure 1. Output Voltage Setting
MAG.LAYERS MCD110C-390K-LV 10 x 6.5 x 10.5
The output voltage is set by an external resistive
voltage divider according to the following equation :
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the
trapezoidal current at the source of the high side
MOSFET. To prevent large ripple current, a low ESR
input capacitor sized for the maximum RMS current
should be used. The approximate RMS current is
given :
R1
R2
V
OUT
= V
1
REF
Where VREF is the reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 0.1F low ESR ceramic capacitor between
the BOOT pin and SW pin. This capacitor provides the
gate driver voltage for the high side MOSFET.
VOUT
V
IN
IRMS = IOUT(MAX)
1
V
IN
VOUT
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 10F low ESR ceramic capacitors are
Suggested. For the Suggested capacitor, please refer
to Table 3 for more details. 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. Loop stability can be checked by viewing
the load transient response as described in a later
section.
Inductor Selection
The inductor value and operating frequency determine
the ripple current according to a specific input and
output voltage. The ripple current IL increases with
higher VIN and decreases with higher inductance.
V
f L
V
OUT
OUT
I =
L
1
V
IN
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 the highest efficiency operation. However,
it requires a large inductor to achieve this goal.
For the ripple current selection, the value of IL =
0.24(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 output ripple, VOUT , is determined by :
1
V
I ESR
OUT
L
8fC
OUT
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6266-00 March 2015
www.richtek.com
9
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