ISL6211 Datasheet, PDF (13/16 Pages) Intersil Corporation – Crusoe™ Processor Core-Voltage Regulator

ISL6211
MOSFET Selection and Considerations
Requirements for the upper and lower MOSFETs are
different in mobile applications. The reason for that is the
10:1 difference in conduction time of the lower and the upper
MOSFETs driven by a difference between the input voltage
which is nominally in the range from 8V to 20V, while
nominal output voltage is usually lower than 1.5V.
Requirements for the lower MOSFET are simpler than
those to the upper one. The lower the Rdson of this device,
the lower the conduction losses, the higher the converter’s
efficiency. Switching losses and gate drive losses are not
significant because of zero-voltage switching conditions
inherent for this device in the buck converter. Low reverse
recovery charge of the body diode is important because it
causes shoot-trough current spikes when the upper
MOSFET turns on. Also, important is to verify that the lower
MOSFET gate voltage does not reach threshold when high
dV/dt transition occurs on the phase node. To minimize this
effect, ISL6211 has a low, 0.8Ω typical, pull-down
resistance of the synchronous rectifier driver.
Requirements for the upper MOSFET Rdson are less
stringent than for the lower MOSFET because its conduction
time is significantly shorter and switching losses are
predominant especially at higher input voltages. It is
recommended to have approximately equal conduction losses
in the lower MOSFET and the switching losses in the upper
MOSFET at the nominal input voltage and load current. Then
the maximum of the converter efficiency is tuned to the
operating point where it is most desired. Also, this provides
the most cost effective solution.
Precise calculation of power dissipation in the MOSFETs is
very complex because many parameters affecting turn-on
and turn-off times such as gate reverse transfer charge, gate
internal resistance, body diode reverse recovery charge,
package and layout impedances and their variation with the
operation conditions are not available to a designer. The
following equations are provided only for rough estimation of
the power losses and should be accompanied by a detailed
breadboard evaluation. Attention should be paid to the input
voltage extremes where power dissipation in the MOSFETs
is usually higher.
Pupper = ^I---^o---²-----^×----^R-----^d----^s----^o---ⁿ-----^×-----^V----^o----^u----^t + ^I---^o----^×-----^V-----ⁱ--ⁿ----^×-----^F----^s-----^×-----⁽--^t--^o----ⁿ-----+-----^t--^o----^f--^f--⁾
Vin
2
Plower
=
I
o²
×
R
d
s
o
n
×


1
–
V_--V---o_--i--un_----t_
Table 2 provides some component information for several
typical applications. Applications 2 and 3 intended for CPUs
other than Transmeta’s Crusoe.
TABLE 2.
APPLICATION APPLICATION APPLICATION
COMPONENT
1
2
3
Maximum CPU
Current
6.0A
12.0A
18.0A
Inductor
1.8µH
1.0µH
0.8µH
Sumida
Panasonic
Panasonic
CEP1231R8MH ETQP6F1R0BFA ETQP6F0R8BFA
Output
Capacitor
4x220µF
6x220µF
6x270µF
Sanyo POSCAP Sanyo POSCAP Panasonic
2R5TPC220M 2R5TPC220M EEFUE0D271R
or
or
3x270µF
5x270µF
Panasonic
Panasonic
EEFUE0271R EEFUE0271R
High-Side
MOSFET
uPA1707
HUF76112SK8
2x
HUF76112SK8
Low-Side
MOSFET
uPA1707
2x
2x
ITF86130SK8T ITF86130SK8T
Current-Input
Resistor for ~3%
Droop
3.57kΩ
2.80kΩ
3.00kΩ
Output Capacitor Selection
The output capacitor serves two major functions in a
switching power supply. Along with the inductor it filters the
sequence of pulses produced by the switcher, and it
supplies the load transient currents. The filtering
requirements are a function of the switching frequency and
the ripple current allowed, and are usually easy to satisfy in
high frequency converters.
The load transient requirements are a function of the slew
rate (di/dt) and the magnitude of the transient load current.
Modern microprocessors produce transient load rates in
excess of 10A/µs. High frequency ceramic capacitors placed
beneath the processor socket initially supply the transient
and reduce the slew rate seen by the bulk capacitors. The
bulk capacitor values are generally determined by the total
allowable ESR rather than actual capacitance requirements.
High frequency decoupling capacitors should be placed as
close to the processor power pins as physically possible.
Consult with the processor manufacturer for specific
decoupling requirements. Use only specialized low-ESR
electrolytic capacitors intended for switching-regulator
applications for the bulk capacitors. The bulk capacitor’s
ESR will determine the output ripple voltage and the initial
voltage drop after a transient. In most cases, multiple
electrolytic capacitors of small case size perform better than
a single large case capacitor.
13