LM3406 Datasheet, PDF (17/37 Pages) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

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LM3406
SW
CS
8V
GND
LM3406, LM3406HV
SNVS512C – SEPTEMBER 2008 – REVISED FEBRUARY 2010
Module
Connector
Module
Connector
~ 0.675
V_TRANSIENT
Figure 18. CS Pin, Transient Path
LM3406
SW
Module
Connector
CS
1k5
8V
GND
Module
Connector
~ 0.675
V_TRANSIENT
Figure 19. CS Pin, Transient Path with Protection
Adding a resistor in series with the CS pin causes the observed output LED current to shift very slightly. The
reason for this is twofold: (1) the CS pin has about 20pF of inherent capacitance inside it which causes a slight
delay (20ns for a 1kΩ series resistor), and (2) the comparator that is watching the voltage at the CS pin uses a
pnp bipolar transistor at its input. The base current of this pnp transistor is approximately 100nA which will cause
a 0.1mV change in the 200mV threshold. These are both very minor changes and are well understood. The shift
in current can either be neglected or taken into consideration by changing the current sense resistance slightly.
CS PIN PROTECTION WITH OVP
When designing output overvoltage protection into the switching converter circuit using a zener diode, transient
protection on the CS pin requires additional consideration. As shown in Figure 20, adding a zener diode from the
output to the CS pin (with the series resistor) for output overvoltage protection will now again allow the transient
energy to be passed through the CS pin’s ESD structure thereby damaging it.
Adding an additional series resistor to the CS pin as shown in Figure 21 will result in the majority of the transient
energy to pass through the sense resistor thereby protecting the LM340X device.
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