ADP8140

Data Sheet

Rev. B | Page 18 of 23

OPERATING THE ADP8140 FROM HIGHER INPUT

VOLTAGES

The ADP8140 is capable of operating from an input voltage

used to power the ADP8140 when an appropriate current

limiting circuit is used.

It is sometimes sufficient to limit the voltage on the VIN pin by

placing a Zener diode on VIN and limiting the current with a

resistor from the input voltage to the VIN pin. This method can

be used if standby power dissipation is not an issue.

Alternatively, if the supply voltage range is small, an additional

Zener diode between the supply and the VIN pin shifts the

voltage at the VIN pin below 30 V. This method adds minimal

power dissipation in both standby and active modes.

However, a more robust voltage limiter uses a Zener diode, an

NPN transistor, and two resistors. This simple circuit, shown in

operation but also reduces the standby current when the

ADP8140 is disabled.

Figure 29. VIN Current Limiting Circuit for High Input Voltages

maximum of the VIN pin. With this circuit, the VIN pin voltage

the current when the ADP8140 is disabled yet still provide

enough current to reverse bias the Zener diode and drive the

NPN transistor when the ADP8140 is active. The current

















Z

Z

CC

RZ

R

V

V

I

few milliwatts of power dissipation during standby.

few hundred ohms is sufficient. When the ADP8140 is active,

the additional worst case power dissipation from this limit

circuitry is given by

(48 V − 24 V + 0.7 V) × 3 mA = 74 mW

The ADP8140 always controls the FB_OUT pin to regulate the

output voltage to provide the minimum amount of headroom

voltage required for the current sinks. One of the current sinks

the other three SINKx pins varies based on the distribution of

the SINKx pins can be statistically calculated. For example,

standard deviation of 70 mV. A statistical analysis of such a

distribution reveals the maximum voltage that may appear on

any of the SINKx pins, as shown in Figure 30). Note that in

Figure 30, the maximum value is defined as the average plus six

standard deviations (σ) of the distribution.

Deviation = 70 mV

The SINKx voltage found on each pin determines the power

that the ADP8140 package must dissipate. Specifically, the

ADP8140 power dissipation can be represented as follows:

(2)

can be performed to predict the total power dissipation within

the ADP8140. For the same distribution used in Equation 2 and

an LED current of 350 mA, Figure 31 gives the average and

maximum power dissipations. Note that in Figure 31, the

maximum value is defined as the average plus six standard

deviations of the distribution.

VIN

ADP8140

NUMBER OF LEDs PER STRING

AVERAGE

MAX

0

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

5

10

15202530