Datasheet

MAX5088/MAX5089
Effective Input Voltage Range
The MAX5088/MAX5089 can operate with input sup-
plies ranging from 4.5V to 5.5V or 5.5V to 23V. The
input voltage range (V+) can be constrained to a mini-
mum by the duty-cycle limitations and to a maximum by
the on-time limitation. The minimum input voltage is
determined by:
D
MAX
is the maximum duty cycle of 87.5% (typ).
V
DROP1
is the total drop in the inductor discharge path
that includes the diode’s forward voltage drop (or the
drop across the synchronous rectifier MOSFET), and
the drops across the series resistance of the inductor
and PC board traces. V
DROP2
is the total drop in the
inductors charging path, which includes the drop
across the internal power MOSFET, and the drops
across the series resistance of the inductor and PC
board traces.
The maximum input voltage can be determined by:
where t
ON_MIN
= 100ns and f
SW
is the switching fre-
quency.
Setting the Output Voltage
For 0.6V or greater output voltages, connect a resistive
divider from V
OUT
to FB to SGND. Select the FB to
SGND resistor (R2) between 1kΩ and 10kΩ and calcu-
late the resistor from OUT to FB (R1) by the following
equation:
where V
FB
= 0.6V, see Figure 3.
For designs that use a Type III compensation scheme,
first calculate R1 for stability requirements (see the
Compensation section) then choose R2 so that:
See Figure 4.
Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX5088/MAX5089: inductance
value (L), peak inductor current (I
PEAK
), and inductor
saturation current (I
SAT
). The minimum required induc-
tance is a function of operating frequency, input-to-out-
put voltage differential, and the peak-to-peak inductor
current (∆I
P-P
). Higher ∆I
P-P
allows for a lower inductor
value, while a lower ∆I
P-P
requires a higher inductor
value. A lower inductor value minimizes size and cost,
improves large-signal and transient response, but
reduces efficiency due to higher peak currents and
higher peak-to-peak output voltage ripple for the same
output capacitor. On the other hand, higher inductance
increases efficiency by reducing the ripple current.
Resistive losses due to extra wire turns can exceed the
benefit gained from lower ripple current levels especial-
ly when the inductance is increased without also allow-
ing for larger inductor dimensions. A good compromise
is to choose ∆I
P-P
equal to 30% of the full load current.
Use the following equation to calculate the inductance:
V
IN
and V
OUT
are typical values so that efficiency is
optimum for typical conditions. The switching frequency
is set by R
OSC
(see the Setting the Switching Frequency
section). The peak-to-peak inductor current, which
reflects the peak-to-peak output ripple, is worse at the
maximum input voltage. See the Output Capacitor
Selection section to verify that the worst-case output rip-
ple is acceptable. The inductor saturation current is also
important to avoid runaway current during continuous
output short-circuit. At high input-to-output differential,
and high switching frequency, the on-time drops to the
order of 100ns. Though the MAX5088/MAX5089 can
control the on-time as low as 100ns, the internal current-
limit circuit may not detect the overcurrent within this
time. In that case, the output current during the fault
may exceed the current limit specified in the EC table.
The overtemperature shutdown protects the
MAX5088/MAX5089 against the output short-circuit
fault. However, the output current may reach 5.5A.
Choose an inductor with a saturation current of greater
than 5.5A when the minimum on-time for a given fre-
quency and duty cycle is less than 200ns.
Input Capacitors
The discontinuous input current of the buck converter
causes large input ripple current. The switching frequen-
cy, peak inductor current, and the allowable peak-to-
peak input voltage ripple dictate the input capacitance
requirement. Increasing the switching frequency or the
inductor value lowers the peak-to-average current ratio
yielding a lower input capacitance requirement.
L
V(VV)
VI
OUT IN OUT
NSW PP
=
−
××
−I
f ∆
R
RV
VV
FB
OUT FB
2
1
=
×
−
RR
V
V
OUT
FB
12 1=× −






V
V
tf
IN MAX
OUT
ON MIN SW
_
_
=
×
V
VV
D
VV
IN MIN
OUT DROP
MAX
DROP DROP_
=
+
+−
1
21
2.2MHz, 2A Buck Converters with an
Integrated High-Side Switch
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