Datasheet

Maxim Integrated │ 14
MAX17503 4.5V-60V, 2.5A, High-Efciency,
Synchronous Step-Down DC-DC Converter
With Internal Compensation
www.maximintegrated.com
Thermal-Shutdown Protection
Thermal-shutdown protection limits total power dissipation
inthedevice.Whenthejunctiontemperatureofthedevice
exceeds +165ºC, an on-chip thermal sensor shuts down
the device, allowing the device to cool. The thermal sensor
turnsthedeviceon again after the junction temperature
cools by 10ºC. Soft-start resets during thermal shutdown.
Carefully evaluate the total power dissipation (see the
Power Dissipation section) to avoid unwanted triggering of
the thermal shutdown in normal operation.
Applications Information
Input Capacitor Selection
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor RMS current requirement (I
RMS
) is
defined by the following equation:
×
= ×
OUT IN OUT
RMS OUT(MAX)
IN
V (V - V )
II
V
where, I
OUT(MAX)
is the maximum load current. I
RMS
has
a maximum value when the input voltage equals twice
the output voltage (V
IN
= 2 x V
OUT
), so I
RMS(MAX)
=
I
OUT(MAX)
/2.
Choose an input capacitor that exhibits less than +10ºC
temperature rise at the RMS input current for optimal
long-termreliability.Uselow-ESRceramiccapacitorswith
high-ripple-currentcapabilityattheinput.X7Rcapacitors
are recommended in industrial applications for their
temperature stability. Calculate the input capacitance
using the following equation:
××
=
η× ×∆
OUT(MAX)
IN
SW IN
I D (1- D)
C
fV
where D = V
OUT
/V
IN
is the duty ratio of the controller,
f
SW
istheswitchingfrequency,ΔV
IN
is the allowable input
voltage ripple, and E is the efficiency.
In applications where the source is located distant from
the device input, an electrolytic capacitor should be
added in parallel to the ceramic capacitor to provide
necessary damping for potential oscillations caused by
the inductance of 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 and output voltage determine the
inductor value as follows:
OUT
SW
V
L
f
=
where V
OUT
, and f
SW
are nominal values.
Select a low-loss inductor closest to the calculated
value with acceptable dimensions and having the lowest
possible DC resistance. The saturation current rating
(I
SAT
) of the inductor must be high enough to ensure that
saturation can occur only above the peak current-limit
value of 3.7A.
Output Capacitor Selection
X7Rceramicoutputcapacitorsarepreferredduetotheir
stability over temperature in industrial applications. The
outputcapacitorsare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. The minimum required 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, DV
OUT
is the allowable
output-voltage deviation, f
C
is the target closed-loop
crossover frequency, and f
SW
is the switching frequency.
Select f
C
to be 1/9th of f
SW
if the switching frequency is
lessthanorequalto500kHz.Iftheswitchingfrequencyis
morethan500kHz,selectf
C
tobe55kHz.
Soft-Start Capacitor Selection
The device implements adjustable soft-start operation to
reduce inrush current. A capacitor connected from the SS pin
toSGNDprogramsthesoft-starttime.Theselectedoutput
capacitance (C
SEL
) and the output voltage (V
OUT
) determine
the minimum required soft-start capacitor as follows:
-6
SS SEL OUT
C 28 10 C V
≥× × ×
The soft-start time (t
SS
) is related to the capacitor
connected at SS (C
SS
) by the following equation:
SS
SS
-6
C
t
5.55 10
=
×
For example, to program a 1ms soft-start time, a 5.6nF
capacitorshouldbeconnectedfromtheSSpintoSGND.