Datasheet

MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
______________________________________________________________________________________ 15
by 10°C, resulting in a pulsed output during continuous
thermal-overload conditions.
During a thermal event, the switching converters are
turned off, POK1 and POK2 are pulled low, and the
soft-starts are reset.
Design Procedure
Output Voltage Setting
The output voltage can be set by a resistive divider net-
work. Select R2, the resistor from FB to GND, between
5kΩ and 15kΩ. Then calculate R1 by:
R1 = R2 x [(V
OUT
/ 0.8) -1]
Inductor Selection
There are several parameters that must be examined
when determining which inductor to use: input voltage,
output voltage, load current, switching frequency, and
LIR. LIR is the ratio of inductor current ripple to DC load
current. A higher LIR value allows for a smaller induc-
tor, but results in higher losses and higher output rip-
ple. A good compromise between size and efficiency is
a 30% LIR. Once all the parameters are chosen, the
inductor value is determined as follows:
where f
S
is the switching frequency. Choose a standard
value close to the calculated value. The exact inductor
value is not critical and can be adjusted in order to
make trade-offs among size, cost, and efficiency.
Lower inductor values minimize size and cost, but also
increase the output ripple and reduce the efficiency
due to higher peak currents. On the other hand, higher
inductor values increase efficiency, but eventually
resistive losses due to extra turns of wire exceed the
benefit gained from lower AC current levels. Find a low-
loss inductor with the lowest possible DC resistance
that fits the allotted dimensions. Ferrite cores are often
the best choice, although powdered iron is inexpensive
and can work well up to 300kHz. The chosen inductor’s
saturation current rating must exceed the peak inductor
current determined as:
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents
defined by the following equation:
Combinations of large electrolytic and small ceramic
capacitors in parallel are recommended. Almost all of
the RMS current is supplied from the large electrolytic
capacitor, while the smaller ceramic capacitor supplies
the fast rise and fall switching edges. Choose the elec-
trolytic capacitor that exhibits less than 10°C tempera-
ture rise at the maximum operating RMS current for
optimum long-term reliability.
Output Capacitor
The key selection parameters for the output capacitor
are the actual capacitance value, the equivalent series
resistance (ESR), the equivalent series inductance
(ESL), and the voltage-rating requirements, which
affect the overall stability, output ripple voltage, and
transient response.
The output ripple has three components: variations in
the charge stored in the output capacitor, voltage drop
across the capacitor’s ESR, and voltage drop across
the capacitor’s ESL, caused by the current into and out
of the capacitor. The following equations estimate the
worst-case ripple:
where I
P-P
is the peak-to-peak inductor current (see the
Inductor Selection section). Higher output current
requires paralleling multiple capacitors to meet the out-
put ripple voltage.
The MAX8537/MAX8538/MAX8539s’ response to a load
transient depends on the selected output capacitor.
After a load transient, the output instantly changes by
(ESR x ∆I
LOAD
) + (ESL x dI/dt). Before the controller
can respond, the output deviates further depending on
the inductor and output capacitor values. After a short
period of time (see the Typical Operating Characteris-
tics), the controller responds by regulating the output
voltage back to its nominal state. The controller
response time depends on the closed-loop bandwidth.
With higher bandwidth, the response time is faster, pre-
VV V V
V I ESR
VICf
V V ESL L ESL
I
VV
fL
V
V
RIPPLE RIPPLE ESR RIPPLE C RIPPLE ESL
RIPPLE ESR P P
RIPPLE C P P OUT SW
RIPPLE ESL IN
PP
IN OUT
SW
OUT
IN
=++
=
=××
=× +
=
−
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
−
−
−
/ ( )
/ ( )
() () ()
()
()
()
8
I
I V VV I V VV
V
RMS
OUT OUT IN OUT OUT OUT IN OUT
IN
=
××− + × ×−[ ( )] [ ( )]
11 12 2 2
22
II
LIR
I
PEAK LOAD MAX LOAD MAX
=+
⎛
⎝
⎜
⎞
⎠
⎟
×
() ()
2
L
VVV
V f I LIR
OUT IN OUT
IN S LOAD MAX
=
×
×× ×
−( )
()