LT3498
16
3498fa
Maximum Output Load Current
The maximum output current of a particular LT3498
circuit is a function of several circuit variables. The fol-
lowing method can be helpful in predicting the maximum
load current for a given circuit:
Step 1: Calculate the peak inductor current:
II
V
L
amps
PK LIMIT
IN
=+
••
–
400 10
9
where I
LIMIT
is 0.3A for the OLED driver. L is the induc-
tance value in Henrys and V
IN
is the input voltage to the
boost circuit.
Step 2: Calculate the inductor ripple current:
I
VV
L
amps
RIPPLE
OUT IN
=
+
()
2
9
1 150 10–••
–
where V
OUT2
is the desired output voltage.
If the inductor ripple current is less then the peak current,
then the circuit will only operate in discontinuous conduc-
tion mode. The inductor value should be increased so
that I
RIPPLE
< I
PK
. An application circuit can be designed
to operate only in discontinuous mode, but the output
current capability will be reduced.
Step 3: Calculate the average input current:
II
I
amps
IN AVG PK
RIPPLE
()
–=
2
Step 4: Calculate the nominal output current:
I
IV
V
amps
OUT NOM
IN AVG IN
OUT
()
()
••.
=
075
2
Step 5: Derate output current:
I
OUT
= I
OUT(NOM)
• 0.7 amps
For low output voltages the output current capability will
be increased. When using output disconnect (load current
taken from V
OUT2
), these higher currents will cause the
drop in the PMOS switch to be higher resulting in reduced
output current capability than those predicted by the
preceding equations.
Inrush Current
When V
IN
is stepped from ground to the operating voltage
while the output capacitor is discharged, a higher level of
inrush current will fl ow through the inductor and integrated
Schottky diode into the output capacitor. Conditions that
increase inrush current include a larger more abrupt voltage
step at V
IN
, a larger output capacitor tied to the CAP2 pin,
and an inductor with a low saturation current. While the
internal diode is designed to handle such events, the inrush
current should not be allowed to exceed 1A. For circuits
that use output capacitor values within the recommended
range and have input voltages of less than 5V, inrush cur-
rent remains low, posing no hazard to the device. In cases
where there are large steps at V
IN
(more than 5V) and/or
a large capacitor is used at the CAP2 pin, inrush current
should be measured to ensure safe operation.
APPLICATIONS INFORMATION — OLED DRIVER
