LM3421 datasheet(14/72 Pages) TI1

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CSH

RCSH

LM3421/23

VCC

RBIAS

RMAX

RADJ

Variable Current Source

Variable

Resistance

VREF

VO = 76V - VIN

VHSP < 76V

VHSN > 3.5V

ILED =

RSNS

1.24V

RSNS

VSNS

RCSH

RHSP

LM3421, LM3421-Q1, LM3423, LM3423-Q1

SNVS574F – JULY 2008 – REVISED JULY 2015

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Feature Description (continued)

(6)

The selection of the three resistors (RSNS, RCSH, and RHSP) is not arbitrary. For matching and noise performance,

the suggested signal current ICSH is approximately 100 µA. This current does not flow in the LEDs and will not

affect either the off-state LED current or the regulated LED current. ICSH can be above or below this value, but

the high-side amplifier offset characteristics may be affected slightly. In addition, to minimize the effect of the

high-side amplifier voltage offset on LED current accuracy, the minimum VSNS is suggested to be 50 mV. Finally,

a resistor (RHSN = RHSP) should be placed in series with the HSN pin to cancel out the effects of the input bias

current (approximately 10 µA) of both inputs of the high-side sense amplifier.

The sense resistor (RSNS) can be placed anywhere in the series string of LEDs as long as the voltage at the HSN

and HSP pins (VHSP and VHSN) satisfies the following conditions.

(7)

Typically, for a buck-boost configuration, RSNS is placed at the bottom of the string (LED-) which allows for

greater flexibility of input and output voltage. However, if there is substantial input voltage ripple allowed, it can

help to place RSNS at the top of the string (LED+) which limits the output voltage of the string to:

(8)

The CSH pin can also be used as a low-side current sense input regulated to 1.24 V. The high-side sense

amplifier is disabled if HSP and HSN are tied to AGND (or VHSN > VHSP) .

7.3.4 Analog Dimming

The CSH pin can be used to analog dim the LED current by adjusting the current sense voltage (VSNS). There

are several different methods to adjust VSNS using the CSH pin:

1. External variable resistance: Adjust a potentiometer placed in series with RCSH to vary VSNS.

2. External variable current source: Source current (0 µA to ICSH) into the CSH pin to adjust VSNS.

Figure 14. Analog Dimming Circuitry

In general, analog dimming applications require a lower switching frequency to minimize the effect of the leading

edge blanking circuit. As the LED current is reduced, the output voltage and the duty cycle decreases.

Eventually, the minimum on-time is reached. The lower the switching frequency, the wider the linear dimming

range. Figure 14 shows how both CSH methods are physically implemented.

Method 1 uses an external potentiometer in the CSH path which is a simple addition to the existing circuitry.

However, the LEDs cannot dim completely because there is always some resistance causing signal current to

flow. This method is also susceptible to noise coupling at the CSH pin because the potentiometer increases the

size of the signal current loop.

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