6

AT9933

In this case V

at low input voltages when input current levels can get too

large. The input current limit must then be designed to limit

the input current to safe levels during input undervoltage

conditions.

Reference

An internally trimmed voltage reference of 1.25V is provided

at the REF pin. The reference can supply a maximum output

current of 500μA 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 Cir-

cuit.

Current Comparators

The AT9933 features two identical comparators with a built-

in 100mV hysteresis. When the GATE is low, the inverting

terminal is connected to 100mV and 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 will be less

than the programmed current and hence, 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 will be in operation

during the steady state operation of the circuit. This com-

parator turns the MOSFET on and off based on the LED

current.

PWM Dimming

PWM Dimming can be achieved by applying a TTL-compat-

ible square wave signal at 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 con-

nected to GND, the gate driver is disabled and the external

MOSFET turns off. The IC is designed so that 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 100Hz to a few kilo

hertz.

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 volt-

age 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.

Application Information

Over-voltage Protection

Over-voltage protection can be added by splitting the output

side resistor R

diode D3 (see the Design Example Circuit on the following

page). When there is an open LED condition, the diode D3

will clamp the output voltage and the zener diode current will

be regulated by the sum of R

Damping Circuit

The Ćuk converter is inherently unstable when the output

current is being controlled. An uncontrolled input current will

lead to an un-damped oscillation between L1 and C1 caus-

ing excessively high voltages across C1. To prevent these

oscillations, a damping circuit consisting of RD and CD is

applied across the capacitor C1. This damping circuit will

stabilize the circuit and help in the proper operation of the

AT9933 based Ćuk converter.

Design and Operation of the Boost-Buck Con-

verter

For details on the design for a Boost-Buck converter using

the AT9933 and the calculation of the damping components,

please refer to Application Note AN-H51.