Datasheet

MAX17502 60V, 1A, Ultra-Small, High-Efciency,
Synchronous Step-Down DC-DC Converter
www.maximintegrated.com
Maxim Integrated │ 12
Prebiased Output
When the device starts into a prebiased output, both the
high-side and low-side switches are turned off so the
converter does not sink current from the output. High-
side and low-side switches do not start switching until
the PWM comparator commands the first PWM pulse, at
which point switching commences first with the high-side
switch. The output voltage is then smoothly ramped up to
the target value in alignment with the internal reference.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa tion
in the device. When the junction temperature of the device
exceeds +165°C, an on-chip thermal sensor shuts down
the device, allowing the device to cool. The thermal sensor
turns the device on again after the junc tion temperature
cools by 10°C. Soft-start resets during thermal shutdown.
Carefully evaluate the total power dissipation (see the
Power Dissipation section) to avoid unwanted triggering of
the thermal-overload protection in normal operation.
Applications Information
Input Capacitor Selection
The discontinuous input-current waveform of the buck
converter causes large ripple currents in the input capaci-
tor. The switching frequency, peak inductor cur rent, and
the allowable peak-to-peak voltage ripple that reflects
back to the source dictate the capacitance requirement.
The device’s high switching frequency allows the use of
smaller value input capacitors. X7R capacitors are rec-
ommended in industrial applications for their temperature
stability.Aminimumvalueof2.2μFshouldbeusedforthe
inputcapacitor.Highervalueshelpreducetherippleon
the input DC bus further. In applications where the source
is located distant from the device input, an electrolytic
capacitorshouldbeaddedinparalleltothe2.2μFceramic
capacitor to provide necessary damping for potential
oscillations caused by the longer input power path and
input ceramic capacitor.
Inductor Selection
Three key inductor parameters must be specified for
operation with the device: inductance value (L), inductor
saturation current (I
SAT
),andDCresistance(R
DCR
). The
switching frequency, input voltage, and output voltage
determine the inductor value as follows:
OUT IN OUT
IN SW
V (V - V )
L
0.3 V f
×
=
××
where V
IN
, V
OUT
, and f
SW
are nominal values. Ensure
that at any operating condition, the ratio (V
OUT
/(L x f
SW
))
is between 300mA and 500mA.
Select a low-loss inductor closest to the calculated value
with acceptable dimensions and having the lowest pos-
sible DC resistance. The saturation current rating (I
SAT
)
of the inductor must be high enough to ensure that satu-
ration can occur only above the peak current-limit value
(I
PEAK-LIMIT
(typ) = 1.65A for the device).
Output Capacitor Selection
X7Rceramicoutputcapacitorsarepreferredduetotheir
stability over temperature in industrial applications. The
output capacitor is usually sized to supporta step load
of 50% of the maximum output current in the application,
so the output-voltage deviation is contained to ±3% of the
output-voltage change.
For fixed 3.3V output voltage versions, connect a mini-
mum of 22μF (1206) capacitor at the output. For fixed
5Voutputvoltageversions,connectaminimumof10μF
(1210) capacitor at the output. For adjustable output volt-
age versions, the output capacitance can be calculated
as follows:
×
= ×
∆
STEP RESPONSE
OUT
OUT
It
1
C
2V
≅+
RESPONSE
C SW
0.33 1
t
ff
where I
STEP
is the load current step, t
RESPONSE
is the
response time of the controller, ΔV
OUT
is the allowable
output-voltage deviation, f
C
is the target closed-loop cross-
over frequency, and f
SW
is the switching frequency. Select
f
C
to be 1/12th of f
SW
. Consider DC bias and aging effects
while selecting the output capacitor.
Soft-Start Capacitor Selection
The MAX17502 implements adjustable soft-start opera-
tion to reduce inrush current. A capacitor con nected from
theSSpintoGNDprogramsthesoft-startperiod.
The soft-start time (t
SS
) is related to the capacitor con-
nected at SS (C
SS
) by the following equation:
SS SS
C 5.55 t= ×
where t
SS
is in milliseconds and C
SS
is in nanofarads. For
example, toprogram a 600μs soft-starttime, a 3300pF
capacitorshouldbeconnectedfromtheSSpintoGND.
Ensure that (C
SEL
x V
OUT
/t
SS
) is less than 300mA,
where C
SEL
is the selected output capacitance.