RT8287A Ver la hoja de datos (PDF) - Richtek Technology
Número de pieza
componentes Descripción
Fabricante
RT8287A Datasheet PDF : 14 Pages
| |||
RT8287A
VIN
Chip Enable
PGOOD
REN
100k
Q1
CIN
R3
100k
CC
1 VIN
BOOT 6
RT8287A
7 EN
SW 2, 3, 4, 5
CBOOT
L
9 PGOOD
11
VCC
FB 8
SS 10
RT
CSS
47nF
AGND GND
14 12, 13, 15 (Exposed Pad)
VOUT
R1
COUT
R2
Figure 3. Enable Control Circuit for Logic Control with Low Voltage
VIN
PGOOD
REN
100k
1 VIN
BOOT 6
CIN
RT8287A
7 EN
SW 2, 3, 4, 5
CBOOT
L
REN2
R3
100k
CC
9 PGOOD
11
VCC
FB 8
SS 10
RT
CSS
47nF
AGND GND
14 12, 13, 15 (Exposed Pad)
VOUT
R1
COUT
R2
Figure 4. The Resistors can be Selected to Set IC Lockout Threshold
Power Good Output
The power good output is an open-drain output and requires
a pull up resistor. When the output voltage is 70% below
its set voltage, PGOOD will be pulled low. It is held low
until the output voltage returns to within the allowed
tolerances once more. During soft-start, PGOOD is actively
held low and only allowed to transition high after soft-start
is over and the output voltage has reached 90% of its set
voltage.
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
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
10
ripple current ΔIL increases with higher VIN and decreases
with higher inductance.
ΔIL
=
⎡
⎢⎣
VOUT
f ×L
⎤
⎥⎦
×
⎡⎢⎣1−
VOUT
VIN
⎤
⎥⎦
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
is a registered trademark of Richtek Technology Corporation.
DS8287A-03 June 2012
Share Link: 