AT9919 datasheet(7/16 Pages) MICROCHIP

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DS20005595A-page 7

AT9919

3.0

APPLICATION INFORMATION

3.1

General Description

The

AT9919

step-down

constant-current

high-brightness LED (HB LED) driver. The device

operates from a 4.5V to 40V input voltage range and

provides the gate drive output to an external N-channel

MOSFET. A high-side current sense resistor sets the

output current, and a dedicated PWM dimming input

(DIM) allows for a wide range of dimming duty ratios.

The PWM dimming could also be achieved by applying

a DC voltage between 0V and 2V to the analog

dimming input (ADIM). In this case, the dimming

frequency can be programmed using a single capacitor

at the RAMP pin. The high-side current sensing

scheme minimizes the number of external components

while delivering LED current with a ±8% accuracy,

using a 1% sense resistor.

3.2

Undervoltage Lockout (UVLO)

The AT9919 includes a 3.7V UVLO with 500 mV

hysteresis. When VIN falls below 3.7V, GATE goes low,

turning off the external N-channel MOSFET. GATE

goes high once VIN is 4.5V or higher.

3.3

5V Regulator

VDD is the output of a 5V regulator capable of sourcing

8 mA. Bypass VDD to GND with a 1 µF capacitor.

3.4

DIM Input

The AT9919 allows dimming with a PWM signal at the

DIM input. A logic level below 0.7V at DIM forces the

GATEOUTPUT low, turning off the LED current. To turn

on the LED current, the logic level at DIM must be at

least 2.2V.

3.5

ADIM and RAMP Inputs

The PWM dimming scheme can also be implemented

by applying an analog control signal to the ADIM pin. If

an analog control signal of 0V~2.0V is applied to ADIM,

the device compares this analog input to a voltage

ramp to pulse width modulate the LED current.

Connecting an external capacitor to RAMP programs

the PWM dimming ramp frequency. See Equation 3-1.

EQUATION 3-1:

fPWM

CRAMP 120k



-----------------------------------------

The DIM and ADIM inputs can be used simultaneously.

In such case, a fPWM(MAX) lower than the frequency of

the dimming signal at DIM must be selected. The

smaller dimming duty cycle of ADIM and DIM will

determine the GATE signal.

When the analog control of PWM dimming feature is

not used, RAMP must be wired to GND and ADIM

should be connected to VDD.

One possible application of the ADIM feature may

include

protection

the

LED

load

from

overtemperature by connecting an NTC thermistor to

ADIM as shown in Figure 3-1.

NTC

VDD

ADIM

GND

AT9919

FIGURE 3-1:

Overtemperature Protection

using ADIM Pin.

3.6

Setting LED Current with the

External Resistor (RSENSE)

The output current in the LED is determined by the

external current sense resistor (RSENSE) connected

between VIN and CS. Disregarding the effect of the

propagation delays, the sense resistor can be

calculated as seen in Equation 3-2.

EQUATION 3-2:

RSENSE

---





VRS HI



VRS LO



ILED

---------------------------------------------







200mV

ILED

-----------------



3.7

Selecting Buck Inductor (L)

The AT9919 regulates the LED output current using an

input comparator with hysteresis. (See Figure 3-2.) As

the current through the inductor ramps up, and the

voltage across the sense resistor reaches the upper

threshold, the voltage at GATE goes low, turning off the

external MOSFET. The MOSFET turns on again when

the inductor current ramps down through the

freewheeling diode until the voltage across the sense

resistor equals the lower threshold.

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