isl8009a Intersil Corporation, isl8009a Datasheet - Page 11

• Manufacturer Part Number: isl8009a
• Description: 1.5a Low Quiescent Current 1.6mhz High Efficiency Synchronous Buck Regulator
• Manufacturer: Intersil Corporation

Applications Information
Output Inductor and Capacitor Selection
To consider steady state and transient operation, ISL8009A
typically uses a 3.3µH output inductor. Higher or lower
inductor values can be used to optimize the total converter
system performance. For example, for higher output voltage
3.3V application, in order to decrease the inductor current
ripple and output voltage ripple, the output inductor value
can be increased. The inductor ripple current can be
expressed in Equation 1:
'I
The inductor’s saturation current rating needs be at least
larger than the peak current. The ISL8009A protects the
typical peak current 2.1A. The saturation current needs to be
over 2.4A for maximum output current application.
ISL8009A uses internal compensation network and the
output capacitor value is dependant on the output voltage.
The ceramic capacitor is recommended to be X5R or X7R.
The recommended minimum output capacitor values are
shown in Table 1.
In Table 1, the minimum output capacitor value is given for
different output voltages to make sure the whole converter
system is stable.
Input Capacitor Selection
The main functions for the input capacitor are to provide
decoupling of the parasitic inductance and to provide filtering
function to prevent the switching current from flowing back to
the battery rail. A 10µF, X5R or X7R ceramic capacitor is a
good starting point for the input capacitor selection.
=
V
-------------------------------------- -
O
TABLE 1. OUTPUT CAPACITOR VALUE vs V
V
x
OUT
L f
§
¨
©
0.8
1.2
1.6
1.8
2.5
3.3
3.6
1
x
–
S
(V)
---------
V
V
IN
O
·
¸
¹
C
11
OUT
10
10
10
10
10
10
10
(µF)
1.0~2.2
1.2~2.2
1.8~2.2
1.8~3.3
1.8~3.3
1.8~4.7
1.8~4.7
L (µH)
OUT
(EQ. 1)
ISL8009A
Output Voltage Setting Resistor Selection
The resistors R
voltage for the adjustable version. The output voltage can be
calculated by Equation 2:
where the 0.8V is the reference voltage.
The voltage divider consists of R
quiescent current by VO/(R
desirable. On the other hand, the VFB pin has leakage
current that will cause error in the output voltage setting. The
leakage current has a typical value of 0.1µA. To minimize the
accuracy impact on the output voltage, select the R
larger than 200k:. For V
short R
Layout Recommendation
The layout is a very important converter design step to make
sure the designed converter works well. For the ISL8009A
buck converter, the power loop is composed of the output
inductor L, the output capacitor C
GND pin. It is necessary to make the power loop as small as
possible.
The heat of the IC is mainly dissipated through the thermal
pad. Maximizing the copper area connected to the thermal
pad is preferable. In addition, a solid ground plane is helpful
for EMI performance. It is recommended to add 5 vias under
the thermal pad connection to the solid ground plane.
V
O
=
0.8
2
and open R
x
§
¨
©
1
+
R
------ -
R
2
2
3
and R
·
¸
¹
3
.
3
O
shown in Figure 24 set the output
= 0.8V, it is recommended to
2
+ R
2
3
OUT
), so larger resistance is
and R
, the LX pin and the
3
and increases the
February 13, 2008
3
no
FN6656.0
(EQ. 2)