Rainbow Electronics MAX15020 - Manual (Page 11)

MAX15020
2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
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Soft-Start (SS)
At startup, after V IN is applied and the UVLO threshold
is reached, a 15µA (typ) current is sourced into the
capacitor (C
SS
) connected from SS to GND forcing the
V SS voltage to ramp up slowly. If V REFIN is set to a DC
voltage or has risen faster than the C SS charge rate,
then V SS will stop rising once it reaches V REFIN . If
V REFIN rises at a slower rate, V SS will follow the V REFIN
voltage rise rate. V OUT rises at the same rate as V SS
since V
FB
follows V
SS
.
Set the soft-start time (t SS ) using following equation:
where t SS is in seconds and C SS is in Farads.
Reference Input and
Output (REFIN, REFOUT)
The MAX15020 features a reference input for the inter-
nal error amplifier. The IC regulates FB to the SS voltage
which is driven by the DC voltage applied to REFIN.
Connect REFIN to REFOUT to use the internal 0.98V ref-
erence. Connect REFIN to a variable DC voltage source
to dynamically control the output voltage. Alternatively,
REFIN can also be driven by a duty-cycle control PWM
source through a lowpass RC filter (Figure 2).
Internal Digital Power Supply (DVREG)
DVREG is the supply input for the internal digital power
supply. The power for DVREG is derived from the out-
put of the internal regulator (REG). Connect a 10Ω
resistor from REG to DVREG. Bypass DVREG to GND
with at least a 1µF ceramic capacitor.
Error Amplifier
The output of the internal error amplifier (COMP) is
available for frequency compensation (see the
Compensation Design section). The inverting input is
FB, the noninverting input is SS, and the output is
COMP. The error amplifier has an 80dB open-loop gain
and a 1.8MHz GBW product. When an external clock is
used, connect FSEL to REG.
Oscillator/Synchronization Input (SYNC)
With SYNC connected to GND, the IC uses the internal
oscillator and switches at a fixed frequency of 300kHz or
500kHz based upon the selection of FSEL. For external
synchronization, drive SYNC with an external clock from
100kHz to 500kHz and connect FSEL to REG. When dri-
ven with an external clock, the device synchronizes to
the rising edge of SYNC.
PWM Comparator/Voltage Feed-Forward
An internal ramp generator is compared against the
output of the error amplifier to generate the PWM sig-
nal. The maximum amplitude of the ramp (V
RAMP ) auto-
matically adjusts to compensate for input voltage and
oscillator frequency changes. This causes the V
IN /
V
RAMP
to be a constant 9V/V across the input voltage
range of 7.5V to 40V and the SYNC frequency range of
100kHz to 500kHz. This simplifies loop compensation
design by allowing large input voltage ranges and
large frequency range selection.
Output Short-Circuit
Protection (Hiccup Mode)
The MAX15020 protects against an output short circuit
by utilizing hiccup-mode protection. In hiccup mode, a
series of sequential cycle-by-cycle current-limit events
cause the part to shut down and restart with a soft-start
sequence. This allows the device to operate with a con-
tinuous output short circuit.
During normal operation, the switch current is measured
cycle-by-cycle. When the current limit is exceeded, the
internal power MOSFET turns off until the next on-cycle
and the hiccup counter increments. If the counter
counts four consecutive overcurrent limit events, the
device discharges the soft-start capacitor and shuts
down for 512 clock periods before restarting with a soft-
start sequence. Each time the power MOSFET turns on
and the device does not exceed the current limit, the
counter is reset.
Thermal-Overload Protection
The MAX15020 features an integrated thermal-over-
load protection. Thermal-overload protection limits the
total power dissipation in the device and protects it in
the event of an extended thermal fault condition. When
the die temperature exceeds +160°C, an internal ther-
mal sensor shuts down the part, turning off the power
MOSFET and allowing the IC to cool. After the temper-
ature falls by 20°C, the part restarts beginning with the
soft-start sequence.
t
V C
A
SS
REFIN SS =
×
15 µ