LMP8645 datasheet(12/16 Pages) NSC

12 / 16 page

30071605

FIGURE 3. Allowed Gains for CASE 2

Also in this case once selected the R

GAIN (Gain) the VSENSE

range is fixed too.

Case 3

CM ≥ VS

The max voltage at the R

G pin is Vout_max, it means that

OUT = VSENSE * RGAIN/RIN ≤ Vout_max where Vout_max is

the maximum allowable output voltage according to the Elec-

trical Tables.So all the R

GAIN resistors which respect the

previous inequality are allowed. The graphical representation

helps in the selection; all the combinations (V

SENSE,

GAIN) below the curves are allowed.

30071606

FIGURE 4. Allowed Gains for CASE 3

Also in this case once selected the R

GAIN (Gain) the VSENSE

range is fixed too.

From the cases showed above a good way to maximize the

output voltage swing of the LMP8645 is to select the max al-

lowable Rgain according to the previous equations. For a

fixed supply voltage and Vsense as the common mode volt-

age increases, the max allowable Rgain increases too.

DRIVING ADC

The input stage of an Analog to Digital converter can be mod-

eled with a resistor and a capacitance versus ground. So if

the voltage source doesn't have a low impedance an error in

the amplitude's measurement will occur. In this case a buffer

is needed to drive the ADC. The LMP8645 has an internal

output buffer able to drive a capacitance load up to 30 pF or

the input stage of an ADC. If required an external low pass

RC filter can be added at the output of the LMP8645 to reduce

the noise and the bandwidth of the current sense. Any other

filter solution which implies a capacitance connected to the

G pin is not suggested due to the high impedance of that pin.

30071631

FIGURE 5. LMP8645 to ADC interface

SENSING CURRENT IN LED DRIVER APPLICATIONS

The LMP8645 is the right choice in the applications which re-

quires high side current sense, such as High Brightness LED

for automotive where the LED's cathode has to be connected

to the case (ground) of the car. In this case the classical low

side current sense with a shunt resistor connected between

the LED's cathode and the case doesn't guarantee the ground

connection. In Figure 6, the LMP8645 monitors the current for

the LM3406 a constant current buck regulator. The LMP8645

is supplied by the internal LDO of the LM3406 thorough the

pin VCC, the current which flows in the LED is programmed

according the following formula: I

F= VCS/(RS*Gain), where

Gain = R

GAIN*Gm and VCS=200 mV. In this application the

current which flows in the HB LED is in the range between

350 mA and 1A, so in order to reduce the power dissipation

on the shunt resistor and have a good accuracy, the R

S should

be in the range between 50 m

Ω and 200 mΩ. In the table

below two examples are analyzed.

F=350mA

F=1A

GAIN

40k

Ω

36k

Ω

77m

Ω

27m

Ω

Dissipated Power 9.5mW

27mW

Total Accuracy

≊5%

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