AT9933
In this case V
DD UVLO
cannot be relied upon to turn off the IC
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.
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.
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.
Application Information
Over-voltage Protection
Over-voltage protection can be added by splitting the output
side resistor R
S2
into two components and adding a zener
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
S2A
and R
CS2
.
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.
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
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.
6
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