ML4903
DESIGN CONSIDERATIONS
This section is a quick-check guide for getting ML4903
circuits up and running, with a special emphasis on
Pentium Pro and Pentium II applications. Unless otherwise
noted, all component designators refer to the circuit
shown in Figure 1.
COMPENSATION
The R and C values connected to the COMP pin for loop
compensation are 100k
W
and 1nF, respectively. These
values yield stable operation and rapid transient response
for a most values of L2 and C
OUT
(1µH to 10µH, 1200µF
to 10,000µF), and will generally not need to be altered. If
changes do need to be made, note that the drive
capability of the transconductance error amplifier is
typically 20µA, its Z
OUT
is 5M
W
, and its unity-gain
crossover frequency is approximately 10 MHz.
INPUT AND OUTPUT CAPACITORS
The input and output capacitors used in conjunction with
the ML4903, especially in Pentium Pro and Pentium II
applications, must be able to meet several criteria:
1. The input capacitors must be able to handle a
relatively high ripple current
2. The output capacitors must have a low Equivalent
Series Resistance (ESR) and Equivalent Series
Inductance (ESL)
3. The output capacitors must be able to hold up the
output during the time that the current through the
buck inductor is slewing to meet a transient load step.
The circuit’s input bypass capacitance should be able to
handle a ripple current equal to 0.5 x I
LOAD
. If the
converter sees load peaks only occasionally, and for less
than 30 seconds at a time during those intervals, then the
aluminum electrolytic or OS-CON
®
input capacitors need
only be sized to accommodate the average output load.
Note that tantalum input capacitors have much less
thermal mass than aluminum electrolytics, so this
relaxation of ripple current requirements may not apply to
them.
During a 30A/µs load transient, it is not practical for a
buck converter to slew the its current fast enough to
regulate the instantaneous output voltage required by this
application. During the first few microseconds following
such a “load step,” the output capacitance of the
converter must act as a passive energy source. In
delivering its energy to the load, the output capacitance
must not introduce any considerable impedance, or its
purpose will be defeated. A total voltage aberration
during load transients of ±5% is allowed for the Pentium
Pro and Pentium II. The voltage transient due to ESL and
ESR is:
D
V
=
ESR
´ D
I
OUT
+
ESL
´
1
!
6
di
dt
"#
$
(1)
For example, assume that the output voltage of the
ML4903 is set to 2.8V. To allow no more than 3% of
D
V
OUT
to be contributed by the ESR (84mV) of the output
capacitance, and 2% by its ESL (56mV), the output ESR
should not exceed:
ESR(MAX)
=
84mV
=
6m
W
14A
(2)
Similarly, the output ESL should be less than or equal to:
ESL(MAX)
=
1
m
s
56mV
=
18nH
.
30A
(3)
Achieving these low values of ESL and ESR is not trivial;
doing so typically requires using multiple high-quality
capacitors in parallel, often with dedicated power and
ground planes to minimize interconnection impedance.
The output capacitance should have a value of > 1500µF
to hold the output voltage relatively constant (< 50mV of
sag) until the current in the buck inductor can catch up
with the change in output current. To meet the ESR and
ESL requirements, the actual output capacitance will
usually be significantly greater than this theoretical
minimum. These capacitors can be of all one type, or a
combination of aluminum electrolytic, OS-CON
®
, and
tantalum devices.
Figures 2(a) and 2(b) show oscilloscope photographs of the
transient response of the circuit shown in Figure 1.
OVERCURRENT PROTECTION
Overcurrent protection for the ML4903 application circuit
can be accomplished either by using a low value sense
resistor placed between the current recirculating rectifier
and ground, or by directly monitoring the voltage drop
across a synchronous rectifier MOSFET (Q3||Q4) during
its conduction period. Using a current sense resistor has
the advantages of accuracy over the entire operating
temperature range, and of allowing the use of a Schottky
diode in place of a synchronous rectifier if the efficiency
loss is acceptable. The disadvantages to using a sense
resistor are higher cost and increased power dissipation.
Sensing across the synchronous rectifier has the
advantages of lower cost and of enhanced protection
against overtemperature conditions (the current limit point
is linearly reduced as the MOSFET temperature rises).
If a current sensing resistor is employed (see Figure 3), the
resistor monitors the inductor current during the buck
converter’s off period. This is the interval during which
current will recirculate through the synchronous rectifier,
or the Schottky diode if no synchronous rectifier is used.
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
