LM3402MRX National Semiconductor, LM3402MRX Datasheet - Page 17

• Manufacturer Part Number: LM3402MRX
• Description: LM3402/LM3402HV 0.5A Constant Current Buck Regulator for Driving High Power LEDs; Package: PSOP; No of Pins: 8; Qty per Container: 2500; Container: Reel
• Manufacturer: National Semiconductor

Adding a resistor in series with the CS pin causes the ob-
served 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 thresh-
old. These are both very minor changes and are well under-
stood. 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 switch-
ing converter circuit using a zener diode, transient protection
FIGURE 7. CS Pin, Transient Path with Protection
17
on the CS pin requires additional consideration. As shown in
Figure
(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
pass through the sense resistor thereby protecting the
LM340X device.
Figure 9
8, adding a zener diode from the output to the CS pin
will result in the majority of the transient energy to
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