RT8286 데이터 시트보기 (PDF) - Richtek Technology
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RT8286 Datasheet PDF : 14 Pages
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RT8286
VIN
Chip Enable
REN
100k
Q1
1 VIN
BOOT 4
CIN
RT8286
SW 2, 3
CBOOT
L
5 EN
7 VCC
CC
FB 6
RT
GND 8, 9 (Exposed Pad)
VOUT
R1
COUT
R2
Figure 3. Enable Control Circuit for Logic Control with Low Voltage
VIN
1 VIN
BOOT 4
REN
CIN
100k
RT8286
SW 2, 3
CBOOT
L
VOUT
REN2
5 EN
7 VCC
CC
FB 6
RT
GND 8, 9 (Exposed Pad)
R1
COUT
R2
Figure 4. The Resistors can be Selected to Set IC Lockout Threshold
To prevent enabling circuit when VIN is smaller than the
VOUT target value, a resistive voltage divider can be placed
between the input voltage and ground and connected to
the EN pin to adjust IC lockout threshold, as shown in
Figure 4. For example, if an 8V output voltage is regulated
from a 12V input voltage, the resistor REN2 can be selected
to set input lockout threshold larger than 8V.
Under Output Voltage Protection-Hiccup Mode
For the IC, Hiccup Mode of Under Voltage Protection (UVP)
is provided. When the FB voltage drops below half of the
feedback reference voltage, VFB, the UVP function will be
triggered and the IC will shut down for a period of time and
then recover automatically. The Hiccup Mode of UVP can
reduce input current in short-circuit conditions.
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔIL increases with higher VIN and decreases
with higher inductance.
ΔIL
=
⎡
⎢⎣
VOUT
f ×L
⎤
⎥⎦
×
⎡⎢⎣1−
VOUT
VIN
⎤
⎥⎦
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
10
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. Highest efficiency operation is achieved by reducing
ripple current at low frequency, but it requires a large
inductor to attain this goal.
For the ripple current selection, the value of ΔIL = 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 a specified maximum, the inductor
value should be chosen according to the following
equation :
L
=
⎡
⎢⎣
f
×
VOUT
ΔIL(MAX)
⎤
⎥⎦
×
⎡⎢⎣1−
VOUT
VIN(MAX)
⎤
⎥⎦
The inductor's current rating (caused a 40°C temperature
rising from 25°C 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 and it is
highly recommended to keep inductor value as close as
possible to the recommended inductor values for each
Vout as shown in Table 1.
is a registered trademark of Richtek Technology Corporation.
DS8286-02 June 2012
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