Datasheet

LT3599
17
3599fe
APPLICATIONS INFORMATION
Loop Compensation
The LT3599 has an internal transconductance error
amplifi er for LED current regulation whose V
C
output
compensates the control loop. During overvoltage, the
V
C
node also compensates the control loop. The external
inductor, output capacitor, and the compensation resistor
and capacitor determine the loop stability. The inductor
and output capacitor are chosen based on performance,
size and cost. The compensation resistor and capacitor
at V
C
are selected to optimize control loop stability. For
typical LED applications, a 10nF compensation capacitor
in series with a 2k resistor at V
C
is adequate.
Thermal Consideration
The LT3599 provides four channels for LED strings with
internal NPN devices serving as constant-current sources.
When LED strings are regulated, the lowest LED pin volt-
age is 0.7V. The higher the programmed LED current, the
more power dissipation in the LT3599. For 100mA LED
programming current with a 100% PWM dimming ratio,
at least 280mW is dissipated within the IC due to current
sources. Thermal calculations shall include the power
dissipation on current sources in addition to conventional
switch DC loss, switch AC loss and input quiescent loss.
For best effi ciency, it is recommended that all channels
have the same number of LEDs, and each string has a
similar voltage drop across the LEDs.
Board Layout Considerations
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To prevent electromagnetic interference (EMI) problems,
proper layout of high frequency switching paths is es-
sential. Minimize the length and area of all traces con-
nected to the switching node pin (SW). Always use a
ground plane under the switching regulator to minimize
interplane coupling. Good grounding is essential in LED
fault detection. Recommended component placement is
shown in Figure 13.
Figure 13. Recommended Component Placement
3599 F13
1
3
4
5
6
7
8
9
10
11
12
13
14
LT3599
LED1
LED2
LED3
LED4
DISABLE4
SHORTLED
NC
OPENLED
NC
I
SET
CTRL
V
OUT
LED +
(V
OUT
)
POWER V
IN
2
V
O_SW
SCHOTTKY DIODE
V
IN
SHDN/UVLO
NC
GND
V
REF
SS
R
T
PWM
NC
SYNC
NC
T
SET
FB
V
C
C
OUT
GROUND
EXPOSED
PAD
(PIN 29)
POWER
GROUND
SOLDER EXPOSED PAD (PIN 29)
TO THE ENTIRE COPPER GROUND
PLANE UNDERNEATH THE DEVICE.
CONNECT MULTIPLE GROUND PLANES
THROUGH VIAS UNDERNEATH THE IC
28
27
26
25
24
23
22
21
20
19
18
17
16
15
BYPASS
CAPACITOR
INDUCTOR
R
R
R
SW
C
VREF
C
VIN
C
SS
R
T
R R
C
C
R
C
C
f