Rainbow Electronics MAX16904 - Manual (Page 9)

If the input voltage is reduced and the device
approaches dropout, it tries to turn on the high-side
FET continuously. To maintain gate charge on the high-
side FET, the BST capacitor must be periodically
recharged. To ensure proper charge on the BST
capacitor when in dropout, the high-side FET is turned
off every 6.5μs and the low-side FET is turned on for
about 150ns. This gives an effective duty cycle
of > 97% and a switching frequency of 150kHz when in
dropout.
Spread-Spectrum Option
The device has an optional spread-spectrum version. If
this option is selected, then the internal operating fre-
quency varies by +6% relative to the internally generat-
ed operating frequency of 2.1MHz (typ). Spread
spectrum is offered to improve EMI performance of the
device. By varying the frequency 6% only in the posi-
tive direction, the device still guarantees that the
2.1MHz frequency does not drop into the AM band limit
of 1.8MHz. Additionally, with the low minimum on-time
of 80ns (typ) no pulse skipping is observed for a 5V
output with 18V input maximum battery voltage in
steady state.
The internal spread spectrum does not interfere with
the external clock applied on the SYNC pin. It is active
only when the device is running with internally generat-
ed switching frequency.
Power-Good (PGOOD)
The device features an open-drain power-good output.
PGOOD is an active-high output that pulls low when the
output voltage is below 91% of its nominal value. The
device is high impedance when the output voltage is
above 93% of its nominal value. Connect a 20k
Ω (typ)
pullup resistor to an external supply or the on-chip BIAS
output.
Overcurrent Protection
The device limits the peak output current to 1.5A (typ).
The accuracy of the current limit is ±15%, which makes
selection of external components very easy. To protect
against short-circuit events, the device shuts off when
OUTS is below 1.5V (typ) and one overcurrent event is
detected. The device attempts a soft-start restart every
30ms and stays off if the short circuit has not been
removed. When the current limit is no longer present, it
reaches the output voltage by following the normal soft-
start sequence. If the device die reaches the thermal
limit of +175°C (typ) during the current-limit event, it
immediately shuts off.
Thermal-Overload Protection
The device features thermal-overload protection. The
device turns off when the junction temperature exceeds
+175°C (typ). Once the device cools by 15°C (typ), it
turns back on with a soft-start sequence.
Applications Information
Inductor Selection
Three key inductor parameters must be specified for
operation with the device: inductance value (L), peak
inductor current (I
PEAK ), and inductor saturation current
(I SAT ). The minimum required inductance is a function
of operating frequency, input-to-output voltage differen-
tial, and the peak-to-peak inductor current
(
ΔI
P-P ). Higher ΔI
P-P allows for a lower inductor value,
while a lower ΔI
P-P requires a higher inductor value. A
lower inductor value minimizes size and cost, improves
large-signal and transient response, but reduces effi-
ciency due to higher peak currents and higher peak-to-
peak output-voltage ripple for the same output
capacitor. On the other hand, higher inductance
increases efficiency by reducing the ripple current.
Resistive losses due to extra wire turns can exceed the
benefit gained from lower ripple current levels especial-
ly when the inductance is increased without also allow-
ing for larger inductor dimensions. A good compromise
is to choose
ΔI
P-P equal to 30% of the full load current.
Use the following equation to calculate the inductance:
V IN and V OUT are typical values so that efficiency is
optimum for typical conditions. The switching frequency
is ~2.1MHz. The peak-to-peak inductor current, which
reflects the peak-to-peak output ripple, is worse at the
maximum input voltage. See the
Output Capacitor sec-
tion to verify that the worst-case output ripple is accept-
able. The inductor saturation current is also important to
avoid runaway current during continuous output short
circuit. The output current may reach 1.725A since this
is the maximum current limit. Choose an inductor with a
saturation current of greater than 1.725A to ensure
proper operation and avoid runaway.
Input Capacitor
The discontinuous input current of the buck converter
causes large input ripple current. The switching frequen-
cy, peak inductor current, and the allowable peak-to-
peak input-voltage ripple dictate the input capacitance
requirement. Increasing the switching frequency or the
L
V V V
V f I
OUT IN OUT
IN SW P P
=
−
Ч Ч
−
( )
Δ
MAX16904
2.1MHz, High-Voltage,
600mA Mini-Buck Converter
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