Datasheet

Thermal-overload protection is intended to protect the
MAX5090 in the event of a fault condition. For normal
circuit operation, do not exceed the absolute maximum
junction temperature rating of T
J
= +150°C.
Setting the Output Voltage
The MAX5090A/MAX5090B have preset output voltages
of 3.3V and 5.0V, respectively. Connect FB to V
OUT
for
the preset output voltage (Figure 1).
The MAX5090C offers an adjustable output voltage. Set
the output voltage with a resistive divider connected from
the circuit’s output to ground (Figure 2). Connect the
center node of the divider to FB. Choose R4 less than
15kΩ, then calculate R3 as follows:
OUT
(V 1.228)
R3 R4
1.228
−
= ×
The MAX5090 features internal compensation for
optimum closed-loop bandwidth and phase margin.
Because of the internal compensation, the output must
be sensed immediately after the primary LC.
Inductor Selection
The MAX5090 is a fixed-frequency converter with fixed
internal frequency compensation. The internal fixed
compensation assumes a 100µH inductor and 100µF
output capacitor with 50mΩ ESR. It relies on the location
of the double LC pole and the ESR zero frequency for
proper closed-loop bandwidth and the phase margin at the
closed-loop unity-gain frequency. See Table 2 for proper
component values. Usually, the choice of an inductor is
guided by the voltage difference between V
IN
and V
OUT
,
the required output current, and the operating frequency
of the circuit. However, use the recommended inductors
in Table 2 to ensure stable operation with optimum band-
width.
Use an inductor with a maximum saturation current rating
greater than or equal to the maximum peak current limit
(5A). Use inductors with low DC resistance for a higher
efficiency converter.
Selecting a Rectier
The MAX5090 requires an external Schottky rectifier as
a freewheeling diode. Connec t this rectifier close to the
device using short leads and short PCB traces. The recti-
fier diode must fully conduct the inductor current when the
power FET is off to have a full rectifier function. Choose a
rectifier with a contin uous current rating greater than the
highest expected output current. Use a rectifier with a
voltage rating greater than the maximum expected input
voltage, V
IN
. Use a low forward-voltage Schottky rectifier
for proper operation and high efficiency. Avoid higher than
necessary reverse-voltage Schottky rectifiers that have
higher forward-voltage drops. Use a Schottky rectifier with
forward-voltage drop (V
F
) less than 0.55V and 0.45V at +25°C
and +125°C, respectively, and at maximum load current to
avoid forward biasing of the internal parasitic body diode
(LX to ground). See Figure 3 for forward-voltage drop vs.
temperature of the internal body diode of the MAX5090.
Internal parasitic body-diode conduction may cause improper
operation, excessive junction temperature rise, and thermal
shutdown. Use Table 1 to choose the proper rectifier at
different input voltages and output current.
Input Bypass Capacitor
The discontinuous input current waveform of the buck
converter causes large ripple currents in the input capaci-
tor. The switching frequency, peak inductor current, and
the allowable peak-to-peak voltage ripple reflecting back
to the source dictate the capacitance requirement. The
MAX5090 high-switching frequency allows the use of
smaller-value input capacitors.
The input ripple is comprised of ∆V
Q
(caused by the
capacitor discharge) and ∆V
ESR
(caused by the ESR
of the capacitor). Use low-ESR aluminum electrolytic
capacitors with high-ripple current capability at the input.
Assuming that the contribution from the ESR and capaci-
tor discharge is equal to 90% and 10%, respectively,
Table 1. Diode Selection
V
IN
(V)
DIODE PART
NUMBER
MANUFACTURER
6.5 to 36
B340LB Diodes Inc.
RB051L-40 Central Semiconductor
MBRS340T3 ON Semiconductor
6.5 to 56
MBRM560 Diodes Inc.
RB095B-60 Central Semiconductor
MBRD360T4 ON Semiconductor
6.5 to 76
50SQ80 IR
PDS5100H Diodes Inc.
MAX5090A/B/C 2A, 76V, High-Efciency MAXPower
Step-Down DC-DC Converters
www.maximintegrated.com
Maxim Integrated │ 11