LT3498
APPLICATIONS INFORMATION—OLED DRIVER
Maximum Output Load Current
Step 4: Calculate the nominal output 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:
IIN(AVG) • VIN • 0.75
VOUT2
IOUT(NOM)
=
amps
Step 5: Derate output current:
= I • 0.7 amps
I
Step 1: Calculate the peak inductor current:
OUT
OUT(NOM)
V • 400 •10–9
For low output voltages the output current capability will
beincreased. Whenusingoutputdisconnect(loadcurrent
IN
IPK = ILIMIT
where I
+
amps
L
taken from V
), these higher currents will cause the
OUT2
is 0.3A for the OLED driver. L is the induc-
drop in the PMOS switch to be higher resulting in reduced
output current capability than those predicted by the
preceding equations.
LIMIT
tance value in Henrys and V is the input voltage to the
IN
boost circuit.
Step 2: Calculate the inductor ripple current:
Inrush Current
V
+ 1– V • 150 • 10–9
(
)
When V is stepped from ground to the operating voltage
OUT2
IN
IN
IRIPPLE
=
amps
while the output capacitor is discharged, a higher level of
inrushcurrentwillflowthroughtheinductorandintegrated
Schottky diode into the output capacitor. Conditions that
increaseinrushcurrentincludealargermoreabruptvoltage
L
where V
is the desired output voltage.
OUT2
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
step at V , a larger output capacitor tied to the CAP2 pin,
IN
and an inductor with a low saturation current. While the
internaldiodeisdesignedtohandlesuchevents,theinrush
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
that I
< I . An application circuit can be designed
RIPPLE
PK
to operate only in discontinuous mode, but the output
current capability will be reduced.
Step 3: Calculate the average input current:
where there are large steps at V (more than 5V) and/or
IN
IRIPPLE
IIN(AVG) = IPK
–
amps
a large capacitor is used at the CAP2 pin, inrush current
2
should be measured to ensure safe operation.
3498f
16
Linear [ Linear ]
