Datasheet

T
dim
DT
dim
t
I
LED(t)
I
F
Board Features www.ti.com
Figure 6. Illustration of Current through LED Stack with PWM Dimming
The LM3423 evaluation board can be configured for either high-side PWM dimming or low-side PWM
dimming. The definition of high side dimming is when a MOSFET that interrupts the forward current
through the LED stack is placed on the anode side of the LED stack. Low side dimming places the
MOSFET on the cathode side of the LED stack. The PWM dimming signal should be applied to either the
BNC connector or test point TP6.
Dimming on the low-side (cathode) of the LED load is enabled using the jumper configuration described in
Table 6.
Table 6. Low-Side PWM Dimming Configuration
Jumper Operation
J5 Open Jumper
5.6 Shutdown Operation
The LM3423 can be configured for either a very low quiescent current shut down (“Zero Current” IQ < 1
µA), or the standard enable/disable configuration (I
Q
< 3 mA).
“Zero Current” is achieved by tying the bottom resistor of all external resistor dividers (i.e. V
IN
UVLO, OVP)
to the RPD Pin 18. Bias currents in the resistor dividers are essentially eliminated during shutdown. The
evaluation board is designed using the “zero” shutdown feature.
5.7 Fault Protection Flag
The LM3423 can be configured with fault protection by using the fault flag indicator FLT (pin 9).
When a fault condition is detected, the FLT pin will go high (pulled up to V
IN
by resistor R2).
5.8 Compensation
The LM3423’s error amplifier (EA) is a transconductance type amplifier, which allows for easy single-pin
compensation. When a capacitor is used on the output of the converter to reduce LED ripple current, a
two pole system results. To offset one of the two poles, and guarantee loop stability, a zero is introduced
at the output of the EA. This takes the form of a resistor in series with a compensation capacitor (R21 and
C5). The value of the EA resistor and capacitor is calculated to give the same RC time constant as the
output capacitor and the dynamic resistance (RD) of the LED string.
(r
D-TOTAL
x C
OUT
) = (R21 x C5) (14)
5.9 LED Dynamic Resistance
When the load is an LED or string of LEDs, the load resistance is replaced with the dynamic resistance
(r
D
) and the current sense resistor. LEDs are PN junction diodes, and their dynamic resistance shifts as
their forward current changes. Dividing V
F
by I
F
leads to incorrect results that are 5 to 10 times higher than
the true r
D
value.
8
AN-1907 LM3423 Buck-Boost Configuration Evaluation Board SNVA376A–December 2008–Revised May 2013
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