SW

LM3404

GND

CS

8V

~ 0.67

5

Module

Connector

Module

Connector

1 k

5

LM3404, LM3404HV

SNVS465F – OCTOBER 2006 – REVISED MAY 2013

www.ti.com

Figure 25. 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 1k

Ω 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 26, 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

Adding an additional series resistor to the CS pin as shown in Figure 27 will result in the majority of the transient

energy to pass through the sense resistor thereby protecting the LM340X device.

20

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