lt3781 Linear Technology Corporation, lt3781 Datasheet - Page 12

• Manufacturer Part Number: lt3781
• Description: Bootstrap Start Dual Transistor Synchronous Forward Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT3781
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 V
bias condition for the optocoupler. Connecting the
optocoupler transistor collector to the local 5V
reduces Miller capacitance effects and maximizes the
bandwidth of the optocoupler. Also, higher optocoupler
current means higher bandwidth, and the 5V
can provide collector currents up to 10mA.
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
12
FB
SENSE
V
pin. A resistor from V
OUT
Figure 2. Optocoupler High-BW Configuration
U
10
5
9
LT3781
5V
V
V
FB
C
REF
U
FB
5V
to ground provides the DC
W
–
+
1.25V
3781 F02
REF
REF
U
supply
supply
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 R
provides current to charge a timing capacitor C
nected from the FSETpin to ground. The oscillator oper-
ates by allowing R
point R
internal to the LT3781. When the voltage across C
pulled down to 1.5V, the FSET pin becomes high imped-
ance, once again allowing R
Figure 3 is a plot of oscillator frequency vs C
is shown below. Typical values for 300kHz operation
(150kHz system frequency) are C
R
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.
FSET
Figure 3. Oscillator Frequency vs. Timing Components
= 51k .
FSET
FSET
is pulled back toward ground by a 2.5K resistor
600
550
500
450
400
350
300
250
200
100
150
, connected from the 5V
20
FSET
30
to charge C
330pF
40
TIMING RESISTOR (k )
150pF
50
FSET
100pF
60
to charge C
200pF
FSET
70
FSET
up to 2.5V at which
80
REF
90
FSET
= 150pF and
3781 F03
pin to FSET,
FSET
100
and R
FSET
.
FSET
con-
FSET
3781f
is