MAX16903 Maxim Integrated Products, MAX16903 Datasheet - Page 9

• Manufacturer Part Number: MAX16903
• Description: 1A Mini-Buck Converter
• Manufacturer: Maxim Integrated Products

DataSheet.in
operating frequency of 2.1MHz (typ). Spread spectrum is
offered to improve EMI performance of the MAX16903. By
varying the frequency 6% only in the positive direction,
the MAX16903 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 MAX16903 is running with internally gen-
erated switching frequency.
The MAX16903 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
20kΩ (typ) pullup resistor to an external supply or the
on-chip BIAS output.
The MAX16903 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 MAX16903 will
shut off when OUTS is below 1.5V (typ) and one over-
current event is detected. The MAX16903 attempts a
soft-start restart every 30ms and stays off if the short cir-
cuit has not been removed. When the current limit is no
longer present, it reaches the output voltage by follow-
ing the normal soft-start sequence. If the MAX16903 die
reaches the thermal limit of 175°C (typ) during the cur-
rent-limit event, it immediately shuts off.
The MAX16903 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.
Three key inductor parameters must be specified for
operation with the MAX16903: inductance value (L),
peak inductor current (I
current (I
function of operating frequency, input-to-output voltage
differential, and the peak-to-peak inductor current
(∆I
while a lower ∆I
lower inductor value minimizes size and cost, improves
P-P
2.1MHz, High-Voltage, 1A Mini-Buck Converter
). Higher ∆I
SAT
). The minimum required inductance is a
Applications Information
P-P
P-P
_______________________________________________________________________________________
Thermal-Overload Protection
requires a higher inductor value. A
allows for a lower inductor value,
PEAK
Overcurrent Protection
Power-Good (PGOOD)
), and inductor saturation
Inductor Selection
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 ∆I
Use the following equation to calculate the inductance:
V
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 Capacitors
section to verify that the worst-case output ripple is
acceptable. 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.
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
inductor value lowers the peak-to-average current ratio
yielding a lower input capacitance requirement.
The input ripple comprises mainly of ∆V
the capacitor discharge) and ∆V
ESR of the input capacitor). The total voltage ripple is
the sum of ∆V
ripple from the ESR and the capacitor discharge is
equal to 50% each. The following equations show the
ESR and capacitor requirement for a target voltage rip-
ple at the input:
IN
and V
OUT
Q
P-P
are typical values so that efficiency is
L
ESR
and ∆V
C
=
equal to 30% of the full load current.
IN
V
V
IN
=
=
OUT
I
⎛
⎝ ⎜
OUT
×
ESR
I
OUT
∆V
(
f
SW
V
IN
Q
∆
. Assume the input-voltage
×
V
+
−
ESR
×
×
D
∆
V
∆
I
1 (
f
OUT
P P
SW
2
I
P P
−
ESR
−
Input Capacitor
−
D
)
⎞
⎠ ⎟
) )
(caused by the
Q
(caused by
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