MAX17503 Datasheet, PDF (14/19 Pages) Maxim Integrated Products – Wide 4.5V to 60V Input Voltage Range

MAX17503
4.5V-60V, 2.5A, High-Efficiency,
Synchronous Step-Down DC-DC Converter
With Internal Compensation
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:
= I_RMS I_OUT(MAX) ×
V_OUT × (V_IN - V_OUT )
V_IN
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:
C_IN
=
I_OUT(MAX) × D × (1-
η × f_SW × ∆V_IN
D)
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:
L = ^VOUT
f_SW
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:
C _OUT=
1 _× I_STEP × t _RESPONSE
2
∆V_OUT
t
RESPONSE
≅
( ^0.33
f_C
+
1
f _sw
)
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:
C_SS ≥ 28 × 10 ^-6 × C_SEL × V_OUT
The soft-start time (t_SS) is related to the capacitor
connected at SS (C_SS) by the following equation:
t _SS
=
C _SS
5.55 × 10 ^-6
For example, to program a 1ms soft-start time, a 5.6nF
capacitor should be connected from the SS pin to SGND.
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