Datasheet

MAX17501 60V, 500mA, Ultra-Small, High-Efciency,
Synchronous Step-Down DC-DC Converter
www.maximintegrated.com
Maxim Integrated │ 14
In hiccup mode, the converter is protected by suspend-
ing switching for a hiccup timeout period of 32,768 clock
cycles. Once the hiccup timeout period expires, soft-start
is attempted again. This operation results in minimal
power dissipation under overload fault conditions.
RESET Output
The device includes a RESET comparator to monitor the
output voltage. The open-drain RESET output requires
an external pullup resistor. RESET can sink 2mA of cur-
rent while low. RESET goes high (high impedance) 1024
switching cycles after the regulator output increases
above 95.5% of the designated nominal regulated volt-
age. RESET goes low when the regulator output voltage
drops to below 92.5% of the nominal regulated voltage.
RESET also goes low during thermal shutdown. RESET
is valid when the device is enabled and V
IN
is above 4.5V.
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1μF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1μF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
0.15 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 150mA and 250mA.
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) = 0.76A 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 and 5V output voltage versions, connect
a minimum of 10μF (1206) capacitor attheoutput. For
adjustable output voltage versions, the output capaci-
tance 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.