LT1103 Datasheet, PDF (14/32 Pages) Linear Technology – Offline Switching Regulator

LT1103/LT1105
OPERATIO
LT1103
The LT1103 is a current mode switcher. Switch duty cycle
is controlled by switch current rather than directly by the
output voltage. Referring to the block diagram, the switch
is turned on at the start of each oscillator cycle. It is turned
off when switch current reaches a predetermined level.
Control of output voltage is obtained by using the output
of a voltage sensing error amplifier to set current trip level.
This technique has several advantages. First, it has imme-
diate response to input voltage variations, unlike ordinary
switchers which have notoriously poor line transient
response. Second, it reduces the 90° phase shift at mid
frequencies in the transformer. This greatly simplifies
closed-loop frequency compensation under widely vary-
ing input voltage or output load conditions. Finally, it
allows simple pulse-by-pulse current limiting to provide
maximum switch protection under output overload or
short-circuit conditions.
A start-up loop with hysteresis allows the IC supply
voltage to be bootstrapped from an extra primary side
winding on the power transformer. From 0V to 16V on V_IN,
the LT1103 is in a prestart mode and total input current is
typically 200µA. Above 16V, up to 30V, the 6V regulator
that biases the internal circuitry and the externally avail-
able 15V regulator is turned on. The internal circuitry
remains biased on until V_IN drops below 7V and the part
returns to the prestart mode. Output switching stops when
the V_SW drive is less than 10V corresponding to V_IN of
about 12V.
The oscillator provides the basic clock for all internal
timing. Frequency is adjustable to 200kHz with one exter-
nal capacitor from OSC to ground. The oscillator turns on
the output switch via the logic and driver circuitry. Adap-
tive antisat circuitry detects the onset of saturation in the
power switch and adjusts driver current instantaneously
to limit switch saturation. This minimizes driver dissipa-
tion and provides very rapid turn-off of the switch.
The LT1103 is designed to drive the source of an external
power FET in common gate configuration. The 15V regu-
lator biases the gate to guarantee the FET is on when the
switch is on. Special drive detection circuitry senses the
gate bias voltage and prevents the output switch from
turning on if the gate voltage is less than 10V or greater
than 20V, the industry standards for power MOSFET
operation.
The switch current is sensed internally and amplified to
trip the comparator and turn off the switch according to
the V_C pin control voltage. A blanking circuit suppresses
the output of the current limit comparator for 500ns at the
beginning of each switch cycle. This prevents false trip-
ping of the comparator due to current spikes caused by
external parasitic capacitance and diode stored charge.
The 4.5V Zener-based reference biases the positive input
of the sampling error amplifier. The negative input (FB) is
used for output voltage sensing. The sampling error
amplifier allows the LT1103 to operate in fully isolated
flyback mode by regulating from the flyback voltage of the
bootstrap winding. The leakage inductance spike at the
leading edge of the flyback waveform is ignored with a
blanking circuit. The flyback waveform is directly propor-
tional to the output voltage in a transformer-coupled
flyback topology. Output voltages are fully floating up to
the breakdown voltage of the transformer windings. Mul-
tiple floating outputs are easily obtained with additional
windings.
The error signal developed at the comparator input is
brought out externally. This V_C pin has three functions
including frequency compensation, current limit adjust-
ment and total regulator shutdown. During normal opera-
tion, this pin sits at a voltage between 1.2V (low output
current) and 4.4V (high output current). The error ampli-
fier is a current output (g_m) type, so this voltage can be
externally clamped for adjusting current limit. Switch duty
cycle goes to zero if the V_C pin is pulled to ground through
a diode, placing the LT1103 in an idle mode. Pulling the V_C
pin below 0.15V causes total regulator shutdown and
places the LT1103 in a prestart mode.
LT1105
The LT1105 is a current mode switcher. Switch duty cycle
is controlled by switch current rather than directly by
output voltage. Referring to the block diagram, the switch
is turned on at the start of each oscillator cycle. It is turned
off when switch current reaches a predetermined level.
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