11
FN6612.0
December 21, 2007
The rising edge of the PG output is delayed by 215ms
(typical) from the time the power-good signal is issued. This
function is provided on ISL9107 only.
Applications Information
Inductor and Output Capacitor Selection
To achieve better steady state and transient response,
typically a 2.2µH inductor can be used. The peak-to-peak
inductor current ripple can be expressed as in Equation 1:
In Equation 1, usually the typical values can be used but to
have a more conservative estimation, the inductance should
consider the value with worst case tolerance; and for
switching frequency (f
S
), the minimum f
S
from the “Electrical
Specifications” table on page 2 can be used.
To select the inductor, its saturation current rating should be
at least higher than the sum of the maximum output current
and half of the delta calculated from Equation 1. Another
more conservative approach is to select the inductor with the
current rating higher than the P-Channel MOSFET peak
current limit.
Another consideration is the inductor DC resistance since it
directly affects the efficiency of the converter. Ideally, the
inductor with the lower DC resistance should be considered
to achieve higher efficiency.
Inductor specifications could be different from different
manufacturers so please check with each manufacturer if
additional information is needed.
For the output capacitor, a ceramic capacitor can be used
because of the low ESR values, which helps to minimize the
output voltage ripple. A typical value of 10µF/6.3V ceramic
capacitor should be enough for most of the applications and
the capacitor should be X5R or X7R.
Input Capacitor Selection
The main function for the input capacitor is 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/6.3V ceramic capacitor (X5R or X7R)
is a good starting point for the input capacitor selection.
Output Voltage Setting Resistor Selection
The voltage divider resistors (R
2
and R
3
), as shown in
Figure 16, set the desired output voltage value. The output
voltage can be calculated using Equation 2:
where V
FB
is the feedback voltage (typically it is 0.8V). The
current flowing through the voltage divider resistors can be
calculated as V
O
/(R
2
+ R
3
), so larger resistance is desirable
to minimize this current. On the other hand, the FB 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
3
no larger than 200kΩ.
C
3
(shown in Figure 16) is highly recommended to be added
for improving stability and achieving better transient
response. C
3
can be calculated using Equation 3:
Table 1 provides the recommended component values for
some output voltage options.
Layout Recommendation
The PCB layout is a very important converter design step to
make sure the designed converter works well, especially
under the high current, high switching frequency condition.
For ISL9107 and ISL9108, the power loop is composed of
the output inductor (L), the output capacitor (C
OUT
), the SW
pin and the GND pin. It is necessary to make the power loop
as small as possible and the connecting traces among them
should be direct, short and wide; the same type of traces
should be used to connect the VIN pin, the input capacitor
C
IN
and its ground. The switching node of the converter, the
SW pin, and the traces connected to this node are very
noisy, so keep the voltage feedback trace and other noise
sensitive traces away from these noisy traces.
The input capacitor should be placed as close as possible to
the VIN pin. The ground of the input and output capacitors
should be connected as close as possible as well.
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.
ΔI
V
O
1
V
O
V
IN
---------
–
⎝⎠
⎜⎟
⎛⎞
•
Lf
S
•
-------------------------------------- -
=
(EQ. 1)
V
O
V
FB
1
R
2
R
3
-------
+
⎝⎠
⎜⎟
⎛⎞
•=
(EQ. 2)
TABLE 1. ISL9107 AND ISL9108 RECOMMENDED CIRCUIT
CONFIGURATION vs V
OUT
V
OUT
(V) L (µH) C
2
(µF) R
2
(kΩ)C
3
(pF) R
3
(kΩ)
0.8 2.2 10 0 N/A 100
1.0 2.2 10 44.2 470 178
1.2 2.2 10 80.6 270 162
1.5 2.2 10 84.5 270 97.6
1.8 2.2 10 100 220 80.6
2.5 2.2~3.3 10~22 100 220 47.5
2.8 2.2~3.3 10~22 100 220 40.2
3.3 2.2~3.3 10~22 102 220 32.4
C
3
1
2 π× R
2
7.3kHz××
---------------------------------------------------- -
=
(EQ. 3)
ISL9107, ISL9108
