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