ISL9187 Intersil Corporation, ISL9187 Datasheet - Page 11

• Manufacturer Part Number: ISL9187
• Description: (ISL9107 / ISL9108) 1.5A 1.6MHz Low Quiescent Current High Efficiency Synchronous Buck Regulator
• Manufacturer: Intersil Corporation

www.DataSheet4U.com
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
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
Figure 16, set the desired output voltage value. The output
voltage can be calculated using Equation 2:
where V
current flowing through the voltage divider resistors can be
V
ΔI
O
=
=
V
-------------------------------------- -
V
O
FB
FB
•
L f
⎛
⎜
⎝
•
1
•
is the feedback voltage (typically it is 0.8V). The
⎛
⎜
⎝
1
–
S
---------
V
+
V
IN
O
R
------ -
R
2
3
⎞
⎟
⎠
⎞
⎟
⎠
S
), the minimum f
11
2
and R
3
S
), as shown in
from the “Electrical
ISL9107, ISL9108
(EQ. 1)
(EQ. 2)
calculated as V
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
C
for improving stability and achieving better transient
response. C
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
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
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.
C
V
3
IN
3
OUT
TABLE 1. ISL9107 AND ISL9108 RECOMMENDED CIRCUIT
(shown in Figure 16) is highly recommended to be added
0.8
1.0
1.2
1.5
1.8
2.5
2.8
3.3
=
and its ground. The switching node of the converter, the
3
---------------------------------------------------- -
2
(V)
no larger than 200kΩ.
×
π
×
2.2~3.3
2.2~3.3
2.2~3.3
3
L (µH)
CONFIGURATION vs V
R
2.2
2.2
2.2
2.2
2.2
can be calculated using Equation 3:
2
1
O
×
/(R
7.3kHz
2
+ R
C
10~22
10~22
10~22
2
10
10
10
10
10
(µF)
3
), so larger resistance is desirable
R
2
44.2
80.6
84.5
100
100
100
102
OUT
0
(kΩ)
C
3
470
270
270
220
220
220
220
N/A
(pF)
OUT
December 21, 2007
), the SW
R
3
97.6
80.6
47.5
40.2
32.4
FN6612.0
(EQ. 3)
100
178
162
(kΩ)