AT9933 datasheet(8/16 Pages) MICROCHIP

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AT9933

DS20005597A-page 8

 2016 Microchip Technology Inc.

In case of input transients that reduce the input voltage

below 8V (e.g. Cold Crank condition in an automotive

system), the VIN pin of the AT9933 can be connected to

the MOSFET drain through a switching diode using a

small (1 nF) capacitor between VIN and GND as long as

the drain voltage does not exceed 75V. Since the drain

of the FET is at a voltage equal to the sum of the input

and output voltages, the IC will still be operational when

the input goes below 8V. Therefore, a larger capacitor

is needed at the VDD pin to supply power to the IC when

the MOSFET is switched on.

In this case, VDD UVLO cannot be relied upon to turn off

the IC at low input voltages when input current levels

can get too large. In such cases, the input current limit

must be chosen to ensure that the input current is set

to a safe level.

3.4

Reference

An internally trimmed voltage reference of 1.25V is

provided at the REF pin. The reference can supply a

maximum output current of 500 µA to drive external

resistor dividers.

This reference can be used to set the current

thresholds of the two comparators as shown in the

Typical Application Circuit section.

3.5

Current Comparators

The AT9933 features two identical comparators with a

built-in 100 mV hysteresis. When the GATE is low, the

inverting terminal is connected to 100 mV, but when the

GATE is high, it is connected to GND. One comparator

is used for the input current control and the other for the

output current control.

The input side hysteretic controller is in operation

during Start-up, Overload and Input Undervoltage

conditions. This ensures that the input current never

exceeds the designed value. During normal operation,

the input current is less than the programmed current.

Therefore, the output of the input side comparator will

be high. The output of the AND gate will then be

dictated by the output current controller.

The output side hysteretic comparator controls the

external MOSFET during Steady state operation of the

circuit. This comparator turns the MOSFET on and off

based on the LED current.

3.6

PWM Dimming

PWM Dimming can be achieved by applying a

TTL-compatible square wave signal to the PWM pin.

When the PWMD pin is pulled high, the gate driver is

enabled and the circuit operates normally. When the

PWMD pin is left open or connected to GND, the gate

driver is disabled and the external MOSFET turns off.

The signal at the PWMD pin inhibits the driver only and

the IC need not go through the entire start-up cycle

each time, ensuring a quick response time for the

output current. The recommended PWM dimming

frequency range is from 100 Hz to a few kilohertz.

The flying capacitor in the Ćuk converter (C1) is initially

charged to the input voltage VDC (through diodes D1

and D2). When the circuit is turned on and reaches

Steady state, the voltage across C1 will be VDC+VO. In

the absence of diode D2, when the circuit is turned off,

capacitor C1 will discharge through the LEDs and the

input voltage source VDC. Thus, during PWM dimming,

if capacitor C1 has to be charged and discharged each

cycle, the transient response of the circuit will be

limited. By adding diode D2, the voltage across

capacitor C1 is held at VDC+VO even when the circuit

is turned off, enabling the circuit to return quickly to its

Steady state (and bypassing the start-up stage) upon

being enabled.

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