LT3781 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT3781' PDF : 20 Pages View PDF
LT3781
APPLICATIO S I FOR ATIO
Another common error amplifier configuration is for
optocoupler use in fully isolated converters with second-
ary side control (Figure 2). In such a system, the dominant
pole for the feedback loop is created at the secondary side
controller, so the error amplifier needs only to translate the
optocoupler information. The bandwidths of the
optocoupler and amplifier should be as high as possible to
simplify system compensation. This high bandwidth
operation is accomplished by using the error amplifier as
a transimpedance amplifier, with the optocoupler transis-
tor emitter providing feedback information directly into
the VFB pin. A resistor from VFB to ground provides the DC
bias condition for the optocoupler. Connecting the
optocoupler transistor collector to the local 5VREF supply
reduces Miller capacitance effects and maximizes the
bandwidth of the optocoupler. Also, higher optocoupler
current means higher bandwidth, and the 5VREF supply
can provide collector currents up to 10mA.
VOUT
SENSE
5VREF
5
5V
VFB
9
VC
10
LT3781
1.25V
3781 F02
Figure 2. Optocoupler High-BW Configuration
Oscillator Frequency Programming and
Synchronization
The LT3781 internal oscillator runs at twice the system
switching frequency. The oscillator output toggles a T
flip-flop, generating a 50% duty cycle pulse that is used
internally as the system clock for the IC. Free-run
frequency for the internal oscillator is programmed via an
RC timing network connected to the FSET pin. A pull-up
resistor RFSET, connected from the 5VREF pin to FSET,
provides current to charge a timing capacitor CFSET con-
nected from the FSETpin to ground. The oscillator oper-
ates by allowing RFSET to charge CFSET up to 2.5V at which
point RFSET is pulled back toward ground by a 2.5K resistor
internal to the LT3781. When the voltage across CFSET is
pulled down to 1.5V, the FSET pin becomes high imped-
ance, once again allowing RFSET to charge CFSET.
Figure 3 is a plot of oscillator frequency vs CFSET and RFSET
is shown below. Typical values for 300kHz operation
(150kHz system frequency) are CFSET = 150pF and
RFSET = 51kΩ.
600
550
500
450
100pF
400
150pF
350
300
200pF
250
330pF
200
150
100
20 30
40 50 60 70 80
TIMING RESISTOR (kΩ)
90 100
3781 F03
Figure 3. Oscillator Frequency vs. Timing Components
Due the relatively fast fall time of the oscillator waveform,
the FSET pin is held at its 1.5V threshold by an internal low
impedance clamp to reduce undershoot error. As a result,
if this pin is externally forced low for any reason, external
current limiting is required to prevent damage to the
LT3781. Continuous source current from the FSET pin
should not exceed 1mA. Putting a 2k resistor in series with
any low impedance pull-down device will assure proper
function and protect the IC from damage.
3781f
12
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