ISL6217 Datasheet, PDF (16/19 Pages) Intersil Corporation – Precision Multi-Phase Buck PWM Precision Multi-Phase Buck PWM Positioning IMVP-IV™ and IMVP-IV+

ISL6217
As per the Intel IMVP-IV^TM and IMVP-IV+^TM specification,
Droop = 0.003 (Ω). Therefore, 25A of full load current
equates to a 0.075V Droop output voltage from the VID
setpoint (Refer to Figure 3 and Figure 9), R_DROOP can be
selected based on R_ISEN which is calculated through
Equation 3, R_(DSON), and Droop as per the Block Diagram or
the following equation:
R_DROOP
= 2.3 ⋅ (Droop) ⋅
R_ISEN
r_(DSON)
(Ω)
M
(EQ. 6)
levels must be supplied by the output capacitance.
Minimizing the response time can minimize the output
capacitance required.
The channel ripple can be reasonably approximated by the
following equation:
ΔI_CH
=
V_IN − V_OUT
F_SW • L
•
V_OUT
V_IN
(EQ. 7)
The total output ripple current can be approximated from
the curves in Figure 12.
Component Selection Guidelines
OUTPUT CAPACITOR SELECTION
Output capacitors are required to filter the output inductor
current ripple and supply the transient load current. The
filtering requirements are a function of the channel switching
frequency and the output ripple current. The load transient
requirements are a function of the slew rate (di/dt) and the
magnitude of the transient load current.
They provide the total ripple current as a function of duty
cycle and number of active channels, normalized to the
parameter KNORM at zero duty cycle,
K _NORM
=
V_OUT
L • F_SW
(EQ. 8)
Where L is the channel inductor value.
The microprocessor used for IMVP-IV™ and IMVP-IV+™
will produce transient load rates as high as 30A/ns. High
frequency, ceramic capacitors are used to supply the initial
transient current and slow the rate-of-change seen by the
bulk capacitors. Bulk filter capacitor values are generally
determined by the ESR (Effective Series Resistance) and
voltage rating requirements rather than actual capacitance
requirements. To meet the stringent requirements of
IMVP-IV™ and IMVP-IV+™, (15) 2.2mF, 0612 “Flip Chip”
high frequency, ceramic capacitors are placed very close to
the Processor power pins, with care being taken not to add
inductance in the circuit board traces that could cancel the
usefulness of these low inductance components.
Specialized low-ESR capacitors, intended for switching
regulator applications, are recommended for the bulk
capacitors. The bulk capacitors ESR and ESL determine the
output ripple voltage and the initial voltage drop following a
high slew-rate transient edge. Recommended are at least
(4) 4V, 220mF Sanyo Sp-Cap capacitors in parallel, or (5)
330mF SP-Cap style capacitors. These capacitors provide
an equivalent ESR of less than 3mOhms. These
components should be laid out very close to the load.
As the sense trace for VSEN may be long and routed close
to switching nodes, a 1.0mF ceramic decoupling capacitor is
located between VSEN and ground at the ISL6217.
Output Inductor Selection
The output inductor is selected to meet the voltage ripple
requirements and minimize the converter response time to a
load transient. In a multi-phase converter topology, the ripple
current of one active channel partially cancels with the other
active channels to reduce the overall ripple current. The
reduction in total output ripple current results in a lower
overall output voltage ripple.
The inductor selected for the power channels determines the
channel ripple current. Increasing the value of inductance
reduces the total output ripple current and total output
voltage ripple; however, increasing the inductance value will
slow the converter response time to a load transient.
One of the parameters limiting the converter response time
to a load transient is the time required to slew the inductor
current from its initial current level to the transient current
level. During this interval, the difference between the two
16
FIGURE 12. OUTPUT RIPPLE CURRENT MULTIPLIER VS
DUTY CYCLE
Find the intersection of the active channel curve and duty
cycle for your particular application. The resulting ripple
current multiplier from the y-axis is then multiplied by the
normalization factor KNORM, to determine the total output
ripple current for the given application.
ΔI_TOTAL = K_NORM • K _CM
(EQ. 9)
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use ceramic capacitors
for the high frequency decoupling, and bulk capacitors to
supply the RMS current. Small ceramic capacitors must be
placed very close to the upper MOSFET to suppress the
voltage induced in the parasitic circuit impedances.
Two important parameters to consider when selecting the
bulk input capacitor are the voltage rating and the RMS
current rating. For reliable operation, select a bulk capacitor
with voltage, and current ratings above the maximum input
voltage and the largest RMS current required by the circuit.
The capacitor voltage rating should be at least 1.25 times
greater than the maximum input voltage and a voltage
rating of 1.5 times is a conservative guideline. The RMS
current requirement for a converter design can be
approximated with the aid of Figure 13.
Follow the curve for the number of active channels in the
converter design. Next determine the worst case duty cycle