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Figure 7. Dynamic Resistance

1 Amp is a typical driving current for 3W LEDs, and the calculation below shows how the dynamic

resistance of a 5W white InGaN was determined at 1A:

ΔV

= 3.85V - 3.48V = 370 mV

ΔI

= 1.5A - 0A = 1.5A

= ΔV

/ ΔI

= 370 mV / 1.5A = 250 mΩ

Dynamic resistances combine in series and parallel like linear resistors, hence for a string of 'n' series-

connected LEDs the total dynamic resistance would be:

-TOTAL = n x r

+ R

SNS

= 5(250 mV) + 100 mΩ = 1.35Ω

Now that we have calculated the dynamic resistance of our LED string, we can calculate the

compensation resistor and capacitor values (C5 and R21).

OUT

= 330 µF and r

= 1.35Ω

-TOTAL x C

OUT

= 1.95Ω x 220 µF = 430E-6

Choose C5 to equal 100 nF, therefore R21 equals 4.32 kΩ

5.10 Overvoltage Protection

An over-voltage protection (OVP) with programmable hysteresis feature is available on the LM3423 to

protect the device from damage when the boosted output voltage goes above a maximum value.

The OVP threshold is set up by the resister divider network of R22 and R20 which is referenced to the

regulated output voltage (VO). The OVP threshold and hysteresis can be programmed completely

independent of each other. OVP hysteresis is accomplished with an internal 23 µA current source that is

switched on and off into the impedance of the OVP set-point resistor divider. When the OVP pin exceeds

1.24V, the current source is activated to instantly raise the voltage at the OVP pin. When the OVP pin

voltage falls below the 1.24V threshold, the current source is turned off, causing the voltage at the OVP

pin to fall.

SNVA376A–December 2008–Revised May 2013 AN-1907 LM3423 Buck-Boost Configuration Evaluation Board

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