ISL8225M
recommended to increase the frequency to lower the inductor
ripple. Please refer to Figure 25 for frequency selection at different
Application Information
Programming the Output Voltage
operating conditions, then refer to Figure 31 to choose R
.
SYNC
The ISL8225M has an internal 0.6V ±0.7% reference voltage.
Programming the output voltage requires a resistor divider (R1
and R2) between the VOUT, VSEN+, and VSEN- pins, as shown in
Figure 18. Please note that the output voltage accuracy is also
dependent on the resistor accuracy of R1 and R2. The user needs
to select a high accuracy resistor (i.e. 0.5%) in order to achieve
the overall output accuracy. The output voltage can be calculated
as shown in Equation 1:
Selection of Input Capacitor
Selection of the input filter capacitor is based on how much
ripple the supply can tolerate on the DC input line. The larger the
capacitor, the less ripple expected, however, consideration
should be given to the higher surge current during power-up. The
ISL8225M provides a soft-start function that controls and limits
the current surge. The value of the input capacitor can be
calculated as shown in Equation 2:
R1
R2
(EQ. 1)
⎛
⎞
⎠
-------
V
= 0.6 × 1 +
I
• D(1 – D)
OUT
⎝
O
V
(EQ. 2)
----------------------------------
=
C
IN(MIN)
• f
P-P SW
Note: It is recommended to use a 1kΩ value for the top resistor,
R1. The value of the bottom resistor for different output voltages
is shown in Table 4.
where:
• C
is the minimum required input capacitance (µF)
IN(MIN)
TABLE 4. VALUE OF BOTTOM RESISTOR FOR DIFFERENT OUTPUT
• I is the output current (A)
O
VOLTAGES (V
vs R2)
OUT
• D is the duty cycle
R1
V
R2
OUT
• V
is the allowable peak-to-peak voltage (V)
is the switching frequency (Hz)
P-P
(Ω)
(V)
(Ω)
• f
1k
1k
1k
1k
1k
1k
1k
1k
1k
1k
1k
0.6
0.8
1.0
1.2
1.5
1.8
2.0
2.5
3.3
5.0
6.0
Open
3.01k
1.50k
1.00k
665
SW
In addition to the bulk capacitance, some low Equivalent Series
Resistance (ESR) ceramic capacitance is recommended to
decouple between the VIN and PGND of each channel. See Table 2
for some recommended capacitors. This capacitance reduces
voltage ringing created by the switching current across parasitic
circuit elements. All these ceramic capacitors should be placed as
closely as possible to the module pins. The estimated RMS current
should be considered in choosing ceramic capacitors.
Io D(1 – D)
491
422
316
--------------------------------
=
(EQ. 3)
I
IN(RMS)
221
η
137
Each 10µF X5R or X7R ceramic capacitor is typically good for 2A
to 3A of RMS ripple current. Refer to the capacitor vendor to
check the RMS current ratings. In a typical 15A output
application for one channel, if the duty cycle is 0.5, it needs at
least three 10µF X5R or X7R ceramic input capacitors.
110
Due to the minimum off-time limit of 410ns, the module has a
maximum output voltage, depending on input voltage. Refer to
Figure 25 for the 5V input voltage limitation.
Selection of Output Capacitors
1100
20V
IN
The ISL8225M is designed for low-output voltage ripple. The
output voltage ripple and transient requirements can be met with
bulk output capacitors (COUT) that have adequately low ESR.
COUT can be a low ESR tantalum capacitor, a low ESR polymer
capacitor, or a ceramic capacitor. The typical capacitance is
330µF, and decoupled ceramic output capacitors are used for
each phase. See Table 1 and Table 2 for more capacitor
information. Internally optimized loop compensation provides
sufficient stability margins for all ceramic capacitor applications,
with a recommended total value of 300µF per phase. Additional
output filtering may be needed if further reduction of output
ripple or dynamic transient spike is required.
1000
15V
IN
900
800
700
600
500
12V
IN
10V
IN
8V
IN
5V
IN
0
1
2
3
4
5
EN/FF Turn ON/OFF
V
(V)
OUT
Each output of the ISL8225M can be turned on/off independently
through the EN/FF pins. For parallel use, tie all EN/FF pins together.
Since this pin has the feed-forward function, the voltage on this pin
can actively adjust the loop gain to be constant for variable input
voltage. Please refer to Table 1 to select the resistor divider for
FIGURE 25. RECOMMENDED FREQUENCY vs V AT V
IN OUT
At higher output voltage, the inductor ripple increases, which makes
both output ripple and inductor power loss higher. Therefore, it is
FN7822.1
January 31, 2013
19
INTERSIL [ Intersil ]
