Datasheet

Maxim Integrated │ 13
MAX17503 4.5V-60V, 2.5A, High-Efciency,
Synchronous Step-Down DC-DC Converter
With Internal Compensation
www.maximintegrated.com
an R-C network has to be connected in parallel to the
resistorconnectedfromRTtoSGND,asshowninFigure
1.ThevaluesofthecomponentsR8andC13are90.9kW
and 220pF, respectively.
Operating Input Voltage Range
The minimum and maximum operating input voltages for
a given output voltage should be calculated as follows:
OUT OUT(MAX) DCR
IN(MIN)
SW(MAX) OFF(MAX)
OUT(MAX)
V (I (R 0.15))
V
1- (f t )
(I 0.175)
+ ×+
=
×
+×
OUT
IN(MAX)
SW(MAX) ON(MIN)
V
V
f t)
=
×
where V
OUT
is the steady-state output voltage, I
OUT (MAX)
isthemaximumloadcurrent,R
DCR
is the DC resistance
of the inductor, f
SW(MAX)
is the maximum switching
frequency, t
OFF-MAX
is the worst-case minimum switch
off-time (160ns), and t
ON-MIN
is the worst-case minimum
switch on-time (135ns).
External Frequency Synchronization (SYNC)
Theinternaloscillatorofthedevicecanbe synchronized
to an external clock signal on the SYNC pin. The external
synchronization clock frequency must be between 1.1
x f
SW
and 1.4 x f
SW
, where f
SW
is the frequency
programmed by the RTresistor.The minimum external
clock pulse-width high should be greater than 50ns. See
the RT AND SYNC section in the Electrical Characteristics
table for details.
Overcurrent Protection/Hiccup Mode
The device is provided with a robust overcurrent protection
scheme that protects the device under overload and
output short-circuit conditions. A cycle-by-cycle peak
current limit turns off the high-side MOSFET whenever
the high-side switch current exceeds an internal limit
of 3.7A (typ). A runaway current limit on the high-side
switch current at 4.3A (typ) protects the device under
high input voltage, short-circuit conditions when there is
insufficient output voltage available to restore the inductor
current that was built up during the ON period of the
step-down converter. One occurrence of the runaway
current limit triggers a hiccup mode. In addition, if due to
a fault condition, feedback voltage drops to 0.58V (typ)
any time after soft-start is complete, and hiccup mode
is triggered. In hiccup mode, the converter is protected
by suspending switching for a hiccup timeout period of
32,768 clock cycles. Once the hiccup timeout period
expires, soft-start is attempted again. Note that when soft-
start is attempted under overload condition, if feedback
voltage does not exceed 0.58V, the device switches at
halftheprogrammedswitchingfrequency.Hiccupmode
of operation ensures low power dissipation under output
short-circuit 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 goes high (high
impedance) 1024 switching cycles after the regulator
output increases above 95% of the designed nominal
regulated voltage. RESET goes low when the regulator
output voltage drops to below 92% of the nominal
regulated voltage. RESET also goes low during thermal
shutdown.
Prebiased Output
When the device starts into a prebiased output, both the
high-side and the low-side switches are turned off so that
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. The output voltage is
then smoothly ramped up to the target value in alignment
with the internal reference.
Table 1. Switching Frequency vs. RT
Resistor
Figure 1. Setting the Switching Frequency
R5
R8
C13
SWITCHING FREQUENCY (kHz) RT RESISTOR (kΩ)
500 Open
100 210
200 102
400 49.9
1000 19.1
2200 8.06