LT1103 Datasheet, PDF (20/32 Pages) Linear Technology – Offline Switching Regulator

LT1103/LT1105
APPLICATIONS INFORMATION
The LT1105 provides a 15V regulated output intended for
driving the totem-pole output stage. It will source 30mA
into a capacitive load with no stability problems. The
output voltage temperature coefficient is 3mV/°C. If V_IN
drops below 17V, the 15V output follows about 2.0V below
V_IN until the part shuts down. If the 15V output is pulled
above 17.5V, it will sink 5mA.
A special circuit in the LT1105 senses the voltage at the
15V regulated output prior to turning on the switch. The
15V regulator drives the totem-pole output stage and the
V_SW pin will pull the gate of the FET very close to the value
of the 15V output when V_SW turns on. Therefore, the 15V
output represents what the gate bias voltage on the FET
will be when the FET is turned on. If the sensed voltage on
the 15V output is less than 10V or greater than 20V, the
circuit prevents the switch from turning on. This protects
the FET from dissipating high power in a nonsaturated
state or from excessive gate-source voltage. The oscillator
continues to run and the net effect is to skip switching
cycles until the gate bias voltage is corrected. One
consequence of the gate bias detection circuit is that the
start-up window is 4V. This influences the size of the
bypass capacitor on V_IN.
V_SW Output (LT1105)
The V_SW pin of the LT1105 is the output of a 1A totem-pole
driver stage. This output stage turns an external power
MOSFET on by pulling its gate high. Break-Before-Make
action of 200ns is built into each switch edge to eliminate
cross-conduction currents. Fast switching times and high
efficiency are obtained by using a low loss output stage
and a special driver loop which automatically adapts base
drive current to the totem-pole low side drive. The key
element in the loop is an extra emitter on the output pull-
down transistor as seen in the block diagram. This emitter
carries no current when the low side transistor collector
is high (unsaturated). In this condition, the driver can
deliver very high base drive to the output transistor for fast
turn-off. When the low side transistor saturates, the extra
emitter acts as a collector of an NPN operating in inverted
mode and pulls base current away from the driver. This
linear feedback loop serves itself to keep the switch just at
the edge of saturation. This results in nearly zero driver
current. The quasisaturation state of the low side switch
permits rapid turn-on of the external FET when V_SW pulls
high.
Fully Isolated Flyback Mode
A unique sampling error amplifier included in the control
loop of the LT1103/LT1105 eliminates the need for an
optoisolator while providing ±1% line and load regulation
in a magnetic flux-sensed flyback converter. In this mode,
the flyback voltage on the primary during “switch off” time
is sensed and regulated. It is difficult to derive a feedback
signal directly from the primary flyback voltage as this
voltage is typically several hundred volts. A dedicated
winding is not required because the bias winding for the
regulator lends itself to flux-sensing. Flux-sensing made
practical simplifies the design of off line power supplies by
minimizing the total number of external components and
reduces the components which must cross the isolation
barrier to one, the transformer. This inherently implies
greater safety and reliability. The transformer must be
optimized for coupling between the bias winding and the
secondary output winding(s) while maintaining the required
isolation and minimizing the parasitic leakage inductances.
Although magnetic flux-sensing has been used in the past,
the technique has exhibited poor output voltage regulation
due to the parasitics present in a transformer coupled
design. Transformers which provide the safety and isolation
as required by various international safety/regulatory
agencies also provide the poorest output voltage regulation.
Solutions to these parasitic elements have been achieved
with the novel sampling error amplifier of the LT1103/
LT1105. A brief review of flyback converter operation and
the problems which create a poorly regulated output will
provide insight on how the sampling error amplifier of the
LT1103/LT1105 addresses the regulation issue of magnetic
flux sensed converters.
The following figure shows a simplified diagram of a
flyback converter using magnetic flux sensing. The major
parasitic elements present in the transformer coupled
design are indicated. The relationships between the primary
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