LT3498
OPERATION—OLED DRIVER
The low noise boost of the LT3498 uses a novel control
scheme to provide high efficiency over a wide range
of output current. In addition, this technique keeps the
switching frequency above the audio band over all load
conditions.
The operation of the part can be better understood by
referring to the Block Diagram. The part senses the
output voltage by monitoring the voltage on the FB2 pin.
The user sets the desired output voltage by choosing the
value of the external topside feedback resistor. The part
incorporates a precision 182kΩ bottom-side feedback
resistor. Assuming that output voltage adjustment is not
used (CTRL2 pin is tied to 1.5V, or greater), the internal
reference (V
REF
= 1.215V) sets the voltage to which FB2
will servo during regulation.
The Switch Control block senses the output of the ampli-
fier and adjusts the switching frequency, as well as other
parameters to achieve regulation. During the start-up of
the circuit, special precautions are taken to ensure that
the inductor current remains under control.
Because the switching frequency is never allowed to fall
below approximately 50kHz, a minimum load must be
present to prevent the output voltage from drifting too
high. This minimum load is automatically generated within
the part via the Shunt Control block. The level of this current
is adaptable, removing itself when not needed to improve
efficiency at higher load levels.
The low-noise boost of the LT3498 also has an integrated
Schottky diode and PMOS output disconnect switch. The
PMOS switch is turned on when the part is enabled. When
the part is in shutdown, the PMOS switch turns off, allowing
the V
OUT2
node to go to ground. This type of disconnect
function is often required in power supplies.
APPLICATIONS INFORMATION— LED DRIVER
Inductor Selection
A 15µH inductor is recommended for most applications
for the LED driver of the LT3498. Although small size and
high efficiency are major concerns, the inductor should
have low core losses at 2.3MHz and low DCR (copper
wire resistance). Some small inductors in this category
are listed in Table 1. The efficiency comparison of different
inductors is shown in Figure 2.
Table 1: Recommended Inductors
L
(µH)
15
15
10
10
15
15
15
MAX
DCR
(Ω)
0.58
1.6
0.3
1.2
0.764
0.80
0.86
CURRENT
RATING
(mA)
300
200
450
225
380
360
680
80
75
70
EFFICIENCY (%)
65
60
55
50
45
0
5
10
LED CURRENT (mA)
15
20
3498 F02
15uH Murata LQH32CN150K53
15uH Murata LQH2MCN150K02
15uH Cooper SD3110-150
15uH Toko D312C
15uH Coilcraft DO3314-153ML
PART
LQH32CN150K53
LQH2MCN150K02
LQH32CN100K53
LQH2MCN100K02
SD3110-150
1001AS-150M
(TYPE D312C)
D03314-153ML
VENDOR
Murata
www.murata.com
Figure 2. Efficiency Comparison of Different Inductors
Capacitor Selection
The small size of ceramic capacitors makes them ideal for
LT3498 LED driver applications. Use only X5R and X7R
types, because they retain their capacitance over wider
temperature ranges than other types, such as Y5V or
Z5U. A 4.7µF input capacitor and a 1µF output capacitor
are sufficient for most applications.
3498f
Cooper
www.cooperet.com
Toko
www.toko.com
Coilcraft
www.coilcraft.com
9
LINER [ LINEAR TECHNOLOGY ]
