MAX15062 Datasheet, PDF (16/23 Pages) Maxim Integrated Products – Step-Down DC-DC Converters

MAX15062
60V, 300mA, Ultra-Small, High-Efficiency,
Synchronous Step-Down DC-DC Converters
Reset Output (RESET)
The device includes an open-drain RESET output to
monitor the output voltage. RESET goes high impedance
2ms after the output rises above 95% of its nominal set
value and pulls low when the output voltage falls below
92% of the set nominal regulated voltage. RESET asserts
low during the hiccup timeout period.
Startup into a Prebiased Output
The device is capable of soft-start into a prebiased out-
put, without discharging the output capacitor in both the
PFM and forced-PWM modes. Such a feature is useful in
applications where digital integrated circuits with multiple
rails are powered.
Operating Input Voltage Range
The maximum operating input voltage is determined by
the minimum controllable on-time and the minimum oper-
ating input voltage is determined by the maximum duty
cycle and circuit voltage drops. The minimum and maxi-
mum operating input voltages for a given output voltage
should be calculated as follows:
V_INMIN
V_OUT
+
(I_OUT × (R_DCR
D_MAX
+
0.5))
+
(I_OUT
× 1.0)
V_INMAX
=
V_OUT
t _ONMIN × f_SW
where V_OUT is the steady-state output voltage, I_OUT is
the maximum load current, R_DCR is the DC resistance of
the inductor, f_SW is the switching frequency (max), D_MAX
is maximum duty cycle (0.9), and t_ONMIN is the worst-
case minimum controllable switch on-time (130ns).
Overcurrent Protection/Hiccup Mode
The device is provided with a robust overcurrent
protection scheme that protects the device under over-
load and output short-circuit conditions. A cycle-by-cycle
peak current limit turns off the high-side MOSFET when-
ever the high-side switch current exceeds an internal limit
of 0.56A (typ). A runaway current limit on the high-side
switch current at 0.66A (typ) protects the device under
high input voltage, and 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, output voltage drops to 65% (typ) of
its nominal value any time after soft-start is complete,
hiccup mode is triggered. In hiccup mode, the converter
is protected by suspending switching for a hiccup timeout
period of 131ms. Once the hiccup timeout period expires,
soft-start is attempted again. Hiccup mode of operation
ensures low power dissipation under output short-circuit
conditions.
Care should be taken in board layout and system wiring
to prevent violation of the absolute maximum rating of the
FB/V_OUT pin under short-circuit conditions. Under such
conditions, it is possible for the ceramic output capacitor
to oscillate with the board or wiring inductance between
the output capacitor or short-circuited load, thereby caus-
ing the absolute maximum rating of FB/V_OUT (-0.3V) to
be exceeded. The parasitic board or wiring inductance
should be minimized and the output voltage waveform
under short-circuit operation should be verified to ensure
the absolute maximum rating of FB/V_OUT is not exceeded.
Thermal Overload Protection
Thermal overload protection limits the total power dis-
sipation in the device. When the junction temperature
exceeds +166°C, an on-chip thermal sensor shuts down
the device, turns off the internal power MOSFETs, allow-
ing the device to cool down. The thermal sensor turns the
device on after the junction temperature cools by 10°C.
Applications Information
Inductor Selection
A low-loss inductor having the lowest possible DC resis-
tance that fits in the allotted dimensions should be selected.
The saturation current (I_SAT) must be high enough to
ensure that saturation cannot occur below the maximum
current-limit value (I_PEAK-LIMIT) of 0.56A (typ). The required
inductance for a given application can be determined from
the following equation:
L = 9.3 x V_OUT
where L is inductance in µH and V_OUT is output voltage.
Once the L value is known, the next step is to select the
right core material. Ferrite and powdered iron are com-
monly available core materials. Ferrite cores have low
core losses and are preferred for high-efficiency designs.
Powdered iron cores have more core losses and are rela-
tively cheaper than ferrite cores. See Table 1 to select the
inductors for typical applications.
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