LT3799I(RevA) 查看數據表（PDF） - Linear Technology
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LT3799I Datasheet PDF : 20 Pages
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LT3799
Operation
SECONDARY
DIODE CURRENT
IPK(sec)
voltage is 120Hz, so the control loop unity gain frequency
needs to be set less than approximately 120Hz. Without a
large amount of energy storage on the secondary side, the
output current is affected by the supply voltage changes,
but the DC component of the output current is accurate.
SWITCH
WAVEFORM
TFLYBACK
TPERIOD
3799 F01
Figure 1. Secondary Diode Current and Switch Waveforms
current and zero during the rest of the cycle. The equation
for expressing the output current is:
IOUT = 0.5 • IPK • N • D´
where D´ is equal to the percentage of the cycle represented
by the flyback time.
The LT3799 has access to both the primary winding cur-
rent, the input to the current comparator, and when the
flyback time starts and ends. Now the output current can
be calculated by averaging a PWM waveform with the
height of the current limit and the duty cycle of the flyback
time over the entire cycle. In the feedback loop previously
described, the input to the integrator is such a waveform.
The integrator adjusts the peak current until the calculated
output current equals the control voltage. If the calculated
output current is low compared to the control pin, the error
amplifier increases the voltage on the COMP+ node, thus
increasing the current comparator input.
When the VIN_SENSE voltage is connected to a resistor
divider of the supply voltage, the current limit is propor-
tional to the supply voltage if COMP+ is held constant.
The output of the error amplifier is multiplied with the
VIN_SENSE pin voltage. If the LT3799 is configured with a
fast control loop, slower changes from the VIN_SENSE pin
will not interfere with the current limit or the output current.
The COMP+ pin will adjust to the changes of the VIN_SENSE.
The only way for the multiplier to function properly is to
set the control loop to be an order of magnitude slower
than the fundamental frequency of the VIN_SENSE signal. In
the offline case, the fundamental frequency of the supply
10
TRIAC Dimming Features
The LT3799 incorporates some special features that aid in
the design of an offline LED current source when used with
a TRIAC dimmer. TRIAC dimmers are not ideal switches
when turned off and allow milliamps of current to flow
through them. This is an issue if used with a low quiescent
part such as the LT3799. Instead of turning the main power
MOSFET off when the TRIAC is off, this power device is
kept on and sinks the current to properly load the TRIAC.
When the TRIAC turns on, the VIN_SENSE pin detects this
and enables the loop, but the current comparator is always
enabled and turns the switch off if it is tripped.
Start-Up
The LT3799 uses a hysteretic start-up to operate from
high offline voltages. A resistor connected to the supply
voltage protects the part from high voltages. This resis-
tor is connected to the VIN pin on the part and also to a
capacitor. When the resistor charges the part up to 23V and
INTVCC is in regulation at 10V, the part begins to charge the
CT pin to 340mV and then starts to switch. The resistor
does not provide power for the part in steady state, but
relies on the capacitor to start-up the part, then the third
winding begins to provide power to the VIN pin along with
the resistor. An internal voltage clamp is attached to the
VIN pin to prevent the resistor current from allowing VIN
to go above the absolute maximum voltage of the pin.
The internal clamp is set at 25V and is capable of 28mA
(typical) of current at room temperature. But, ideally, the
resistor connected between the input supply and the VIN
pin should be chosen so that less than 10mA is being
shunted by this internal clamp.
CT Pin and Faults
The CT pin is a timing pin for the fault circuitry. When the
input voltages are at the correct levels, the CT pin sources
10µA of current. When the CT pin reaches 340mV, the part
3799fa
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