ISL6219A_07 Datasheet, PDF (16/17 Pages) Intersil Corporation – Microprocessor CORE Voltage Regulator Precision Multi-Phase BUCK PWM Controller for Mobile Applications

ISL6219A
Switching Frequency
There are a number of variables to consider when choosing the
switching frequency. There are considerable effects on the
upper-MOSFET loss calculation and, to a lesser extent, the
lower-MOSFET loss calculation. These effects are outlined in
MOSFETs, and they establish the upper limit for the switching
frequency. The lower limit is established by the requirement for
fast transient response and small output-voltage ripple as
outlined in Output Filter Design. Choose the lowest switching
frequency that allows the regulator to meet the transient-
response requirements. Switching frequency is determined by
the selection of the frequency-setting resistor, RT (see the
figure Typical Application on page 3). Figure 13 and Equation
23 are provided to assist in the selecting the correct value for
RT.
RT = 10^{[11.09 – 1.13log(fS)]}
(EQ. 23)
Figures 14 and 15 can be used to determine the input-capacitor
rms current as of duty cycle, maximum sustained output current
(I_O), and the ratio of the combined peak-to-peak inductor
current (I_L,PP) to I_O. Figure 16 is provided as a reference to
demonstrate the dramatic reductions in input-capacitor rms
current upon the implementation of the multiphase topology.
Input Capacitor Selection
The input capacitors are responsible for sourcing the ac
component of the input current flowing into the upper
MOSFETs. Their rms current capacity must be sufficient to
handle the ac component of the current drawn by the upper
MOSFETs which is related to duty cycle and the number of
active phases.
Figures 14 and 15 can be used to determine the input-capacitor
RMS current as of duty cycle, maximum sustained output
current (I_O), and the ratio of the combined peak-to-peak
inductor current (I_L,PP) to I_O. Figure 16 is provided as a
reference to demonstrate the dramatic reductions in input-
capacitor rms current upon the implementation of the
multiphase topology.
1000
0.3
0.2
0.1
I_PP = 0
I_PP = 0.5 I_O
I_PP = 0.75 I_O
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O /V_IN)
FIGURE 14. NORMALIZED INPUT-CAPACITOR RMS
CURRENT VS DUTY CYCLE FOR 2-PHASE
CONVERTER
0.3 _{IPP = 0}
I_PP = 0.25 I_O
I_PP = 0.5 I_O
I_PP = 0.75 I_O
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_IN / V_O)
FIGURE 15. NORMALIZED INPUT-CAPACITOR RMS
CURRENT VS DUTY CYCLE FOR 3-PHASE
0.6
0.4
100
10
10
100
1000
SWITCHING FREQUENCY (KHZ)
FIGURE 13. RT VS SWITCHING FREQUENCY
10000
16
0.2
I_PP = 0
I_PP = 0.5 I_O
I_PP = 0.75 I_O
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_IN / V_O)
FIGURE 16. NORMALIZED INPUT-CAPACITOR RMS
CURRENT VS DUTY CYCLE FOR SINGLE-PHASE
CONVERTER
FN9093.1
March 20, 2007