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參數(shù)資料
| 型號： | AT9933LG-G |
| 廠商： | SUPERTEX INC |
| 元件分類： | 穩(wěn)壓器 |
| 中文描述： | SWITCHING CONTROLLER, PDSO8 |
| 封裝： | 4.90 X 3.90 MM, 1.75 MM HEIGHT, 1.27 MM PITCH, GREEN, MS-012AA, SOIC-8 |
| 文件頁數(shù)： | 6/9頁 |
| 文件大小： | 505K |
| 代理商： | AT9933LG-G |
6
AT9933
Supertex inc.
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
●
Tel: 408-222-8888
●
www.supertex.com
In this case V
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.
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 uk converter (C1) is initially
charged to the input voltage VDC (through diodes D
and D
).
When the circuit is turned on and reaches steady state, the
voltage across C
will be VDC+VO. In the absence of diode
D
, when the circuit is turned off, capacitor C
will discharge
through the LEDs and the input voltage source VDC. Thus,
during PWM dimming, if capacitor C
has to be charged and
discharged each cycle, the transient response of the circuit
will be limited. By adding diode D
, the voltage across capac-
itor C
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
into two components and adding a zener
diode D
(see the Design Example Circuit on the following
page). When there is an open LED condition, the diode D
3
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 uk converter is inherently unstable when the output
current is being controlled. An uncontrolled input current will
lead to an un-damped oscillation between L
and C
causing
excessively high voltages across C
. To prevent these oscil-
lations, a damping circuit consisting of R
and C
is applied
across the capacitor C
. This damping circuit will stabilize
the circuit and help in the proper operation of the AT9933
based uk converter.
Design and Operation of the Boost-Buck
Converter
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 and AN-H58.
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