MAX16903 Datasheet, PDF (9/13 Pages) Maxim Integrated Products – 2.1MHz, High-Voltage, 1A Mini-Buck Converter Thermal Shutdown Protection

2.1MHz, High-Voltage, 1A Mini-Buck Converter
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.
Power-Good (PGOOD)
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. PGOOD is high impedance when the output volt-
age is above 93% of its nominal value. Connect a 20kΩ
(typ) pullup resistor to an external supply or the on-chip
BIAS output.
Overcurrent Protection
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.
Thermal-Overload Protection
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.
Applications Information
Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX16903: 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
differential, 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:
L = ^{VOUT (VIN − VOUT )}
V_IN × f_SW × ΔI_P−P
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 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
1.75A since this is the maximum current limit. Choose
an inductor with a saturation current of greater than
1.75A 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
inductor value lowers the peak-to-average current ratio
yielding a lower input capacitance requirement.
The input ripple comprises mainly of ΔV_Q (caused by
the capacitor discharge) and ΔV_ESR (caused by the
ESR of the input capacitor). The total voltage ripple is
the sum of ΔV_Q and ΔV_ESR. Assume the input-voltage
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:
ESR
=
ΔV_ESR
⎛
⎝⎜
I_OUT
+
ΔI_{P − P}
2
⎞
⎠⎟
C_IN
=
I_OUT × D(1− D)
ΔV_Q × f_SW
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