ISL6316 Datasheet, PDF (27/29 Pages) Intersil Corporation – Enhanced 4-Phase PWM Controller with 6-Bit VID Code Capable of Precision RDS(ON) or DCR Differential Current Sensing for VR10 Application

ISL6316
Input Supply Voltage Selection
The VCC input of the ISL6316 can be connected either
directly to a +5V supply or through a current limiting resistor to
a +12V supply. An integrated 5.8V shunt regulator maintains
the voltage on the VCC pin when a +12V supply is used. A
300Ω resistor is suggested for limiting the current into the
VCC pin to a worst-case maximum of approximately 25mA.
Switching Frequency
There are a number of variables to consider when choosing
the switching frequency, as there are considerable effects on
the upper-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, R_T (see the figures labeled Typical
Application on pages 4, 5, 6 and 7). Equation 38 is provided to
assist in selecting the correct value for R_T.
R_T
=
2----.--5---X-----1---0----¹_---⁰_-
F_S
(EQ. 38)
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.
0.3
0.2
0.3 _{IL,P-P = 0}
I_L,P-P = 0.25 I_O
0.2
I_L,P-P = 0.5 I_O
I_L,P-P = 0.75 I_O
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 22. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 3-PHASE CONVERTER
For a two phase design, use Figure 21 to determine the input-
capacitor RMS current requirement given the duty cycle,
maximum sustained output current (I_O), and the ratio of the
per-phase peak-to-peak inductor current (I_L,P-P) to I_O. Select
a bulk capacitor with a ripple current rating which will minimize
the total number of input capacitors required to support the
RMS current calculated. The voltage rating of the capacitors
should also be at least 1.25 times greater than the maximum
input voltage.
Figures 22 and 23 provide the same input RMS current
information for three and four phase designs respectively. Use
the same approach to selecting the bulk capacitor type and
number as described above.
Low capacitance, high-frequency ceramic capacitors are
needed in addition to the bulk capacitors to suppress leading
and falling edge voltage spikes. The result from the high
current slew rates produced by the upper MOSFETs turn on
and off. Select low ESL ceramic capacitors and place one as
close as possible to each upper MOSFET drain to minimize
board parasitic impedances and maximize suppression.
0.1
I_L,P-P = 0
I_L,P-P = 0.5 I_O
I_L,P-P = 0.75 I_O
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 21. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 2-PHASE CONVERTER
27
FN9227.0
August 31, 2005