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參數(shù)資料
| 型號(hào)： | UCC35706DTR |
| 廠商： | Texas Instruments, Inc. |
| 英文描述： | HIGH - SPEED VOLTAGE MODE PULSE MODE WIDTH MODULATOR |
| 中文描述： | 高-高速電壓模式脈沖寬度調(diào)制模式 |
| 文件頁(yè)數(shù)： | 4/18頁(yè) |
| 文件大小： | 353K |
| 代理商： | UCC35706DTR |
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SLUS473A
–
NOVEMBER 1999
–
REVISED MARCH 2001
4
www.ti.com
pin descriptions
DISCH:
A resistor to VIN sets the oscillator discharge current programming a maximum duty cycle. When grounded,
an internal comparator switches the oscillator to a quick discharge mode. A small 100-pF capacitor between DISCH
and GND may reduce oscillator jitter without impacting feed-forward performance. I
DISCH
must be between 25
μ
A
and 250
μ
A over the entire V
IN
range.
FB:
Input to the PWM comparator. This pin is intended to interface with an optocoupler. Input impedance is 50-k
typical.
GND:
Ground return pin.
ILIM:
Provides a pulse-by-pulse current limit by terminating the PWM pulse when the input is above 200 mV. This
provides a high speed (25 ns typical) path to reset the PWM latch, allowing for a pulse-by-pulse current limit.
OUT:
The output is intended to drive an external FET driver or other high impedance circuits, but is not intended to
directly drive a power MOSFET. This improves the controller
’
s noise immunity. The output resistance of the PWM
controller, typically 60
pull-up and 30
pull-down, will result in excessive rise and fall times if a power MOSFET
is directly driven at the speeds for which the UCC35705/6 is optimized.
RC:
The oscillator can be configured to provide a maximum duty cycle clamp. In this mode the on
–
time is set by RT
and CT, while the off-time is set by RDISCH and CT. Since the voltage ramp on CT is proportional to VIN, feed-forward
action is obtained. Since the peak oscillator voltage is also proportional to VIN, constant frequency operation is
maintained over the full power supply input range. When the DISCH pin is grounded, the duty cycle clamp is disabled.
The RC pin then provides a low impedance path to ground CT during the off time.
VDD:
Power supply pin. This pin should be bypassed with a 0.1-
μ
F capacitor for proper operation. The undervoltage
lockout function of the UCC35705/6 allows for a low current startup mode and ensures that all circuits become active
in a known state. The UVLO thresholds on the UCC35705 are appropriate for a dc-to-dc converter application. The
wider UVLO hysteresis of the UCC35706 (typically 4 V) is optimized for a bootstrap startup mode from a high
impedance source.
VFF:
The feed-forward pin provides the controller with a voltage proportional to the power supply input voltage. When
the oscillator is providing a duty cycle clamp, a current of 2
×
I
DISCH
is sourced from the VFF pin. A single resistor
RFF between VFF and GND then set VFF to:
VFF
VIN
2
R
FF
R
DISCH
2
R
FF
When the DISCH pin is grounded and the duty cycle clamp is not used, the internal current source is disabled and
a resistor divider from VIN is used to set VFF. In either case, when the voltage on VFF is less than 1.0 V, both the
output and oscillator are disabled.
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