Datasheet

MAX16903
2.1MHz, High-Voltage, 1A Mini-Buck Converter
9
Maxim Integrated
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
The nominal inductor value can be calculated using
Table 1 based on the nominal output voltage of the
device. Select the nearest standard inductance value to
the calculated nominal value. The nominal standard
value selected should be within ±25% of L
NOM
for best
performance.
Input Capacitor
A low-ESR ceramic input capacitor of 1μF or larger is
needed for proper device operation. This value may
need to be larger based on application input-voltage
ripple requirements.
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
V
I
I
C
IDD
ESR
OUT
PP
IN
OUT
=
+
⎛
⎝
⎜
⎞
⎠
⎟
=
×−
−
Δ
Δ
2
1(
))
ΔVf
QSW
×
V
OUT
(V) L
NOM
(μH)
1.8 to 3.1
V
OUT
/0.55
3.2 to 6.5
V
OUT
/0.96
6.6 to 8.1
V
OUT
/1.40
8.2 to 10
V
OUT
/1.75
Table 1. Nominal Output Voltage Values
V
OUT
(V)
CALCULATED
L
NOM
(μH)
STANDARD
VALUE (μH)
1.8 3.3 3.3
3.3 3.4 3.3
5.0 5.2 4.7
8.0 5.7 5.6
Table 2. Examples for Standard Output
Voltages