ISL6217A_14 Datasheet, PDF (18/20 Pages) Intersil Corporation – Precision Multi-Phase Buck PWM Controller for Intel, Mobile Voltage Positioning IMVP-IV and IMVP-IV+

ISL6217A
value, by the current multiplier value found, and the result is
the RMS input current which must be supported by the input
capacitors.
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. 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.
∆I_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
for the converter and find the intersection of this value and
the active channel curve. The worst case duty cycle is
defined as the maximum operating CORE output voltage
divided by the minimum operating battery voltage. Find the
corresponding y-axis value, which is the current multiplier.
Multiply the total full load output current, not the channel
18
FIGURE 13. INPUT RMS RIPPLE CURRENT MULTIPLIER
MOSFET Selection and Considerations
For the Intel IMVP-IV™ and IMVP-IV+™ application, which
requires up to 25A of current, it is suggested that 2 channel
operation with (3) MOSFETs per channel be implemented.
This configuration would be: (1) High Switching Frequency,
Low Gate Charge MOSFET for the Upper, and (2) Low
r_DS(ON) MOSFETs for the Lowers.
In high-current PWM applications, the MOSFET power
dissipation, package selection and heatsink are the
dominant design factors. The power dissipation includes two
loss components: conduction loss and switching loss. These
losses are distributed between the upper and lower
MOSFETs according to duty cycle of the converter. Refer to
the PUPPER and PLOWER equations below. The
conduction losses are the main component of power
dissipation for the lower MOSFETs. Only the upper
MOSFETs have significant switching losses, since the lower
devices turn on and off into near zero voltage. The following
equations assume linear voltage-current transitions and do
not model power loss due to the reverse-recovery of the
lower MOSFETs body diode. The gate-charge losses are
dissipated in the ISL6217A drivers and do not heat the
MOSFETs; however, large gate-charge increases the
switching time t_SW, which increases the upper MOSFET
switching losses. Ensure that both MOSFETs are within their
maximum junction temperature, at high ambient
temperature, by calculating the temperature rise according
to package thermal-resistance specifications.
P_LOWER
=
I_O2
× r_DS(ON) × (V_IN
V_IN
−
V_OUT )
(EQ. 10)
FN9107.3
June 30, 2005