rt8288a Richtek Technology Corporation, rt8288a Datasheet - Page 10

• Manufacturer Part Number: rt8288a
• Description: 4a, 21v 500khz Synchronous Step-down Converter
• Manufacturer: Richtek Technology Corporation

RT8288A
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, V
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 ΔI
with higher inductance.
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 ΔI
(I
current occurs at the highest V
ripple current stays below a specified maximum, the
www.richtek.com
10
Δ
MAX
I =
L
) will be a reasonable starting point. The largest ripple
⎡
⎢
⎣
V
f L
OUT
×
⎤ ⎡
⎥ ⎢
⎦ ⎣
×
L
1
increases with higher V
−
V
V
OUT
IN
Chip Enable
Figure 4. The Resistors can be Selected to Set IC Lockout Threshold
V
IN
Figure 3. Enable Control Circuit for Logic Control with Low Voltage
⎤
⎥
⎦
V
IN
FB
IN
, the UVP function will be
100k
R
R
. To guarantee that the
EN
EN2
Q1
100k
R
EN
C
IN
C
C
and decreases
IN
C
C
IN
C
1
5
7
1
5
7
L
VIN
VCC
EN
VIN
VCC
EN
= 0.24
RT8288A
RT8288A
BOOT
BOOT
GND
GND
SW
SW
FB
FB
inductor value should be chosen according to the following
equation :
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
V
Input and Output Capacitors Selection
The input capacitance, C
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
RMS current is given by :
L =
Component Supplier
OUT
4
2, 3
6
8, 9 (Exposed Pad)
4
2, 3
6
8, 9 (Exposed Pad)
TAIYO YUDEN
⎡
⎢
⎣
f
as shown in Table 1.
× Δ
Table 2. Suggested Inductors for Typical
V
C
C
OUT
I
TDK
TDK
R
BOOT
BOOT
L(MAX)
R
T
T
L
L
⎤ ⎡
⎥ ⎢
⎦ ⎣
×
Application Circuit
1
R1
R2
R1
R2
−
V
VLF10045
SLF12565
IN(MAX)
V
NR8040
Series
OUT
C
C
OUT
OUT
IN
V
, is needed to filter the
OUT
V
DS8288A-01 August 2011
OUT
⎤
⎥
⎦
Dimensions (mm)
12.5 x 12.5 x 6.5
10 x 9.7 x 4.5
8 x 8 x 4