LT3798
8
3798fa
The LT3798 is a current mode switching controller IC
designed specifically for generating a constant current/
constant voltage supply in an isolated flyback topology.
The special problem normally encountered in such circuits
is that information relating to the output voltage and cur-
rent on the isolated secondary side of the transformer
must be communicated to the primary side in order to
maintain regulation. Historically, this has been done with
an opto-isolator. The LT3798 uses a novel method of using
the external MOSFETs peak current information from the
sense resistor to calculate the output current of a flyback
converter without the need of an opto-coupler.
Active power factor correction is becoming a requirement
for offline power supplies and the power levels are de-
creasing. A power factor of one is achieved if the current
drawn is proportional to the input voltage. The LT3798
modulates the peak current limit with a scaled version
of the input voltage. This technique can provide power
factors of 0.97 or greater.
The Block Diagram shows an overall view of the system. The
external components are in a flyback topology configura-
tion. The third winding senses the output voltage and also
supplies power to the part in steady-state operation. The
V
IN
pin supplies power to an internal LDO that generates
10V at the INTV
CC
pin. The novel control circuitry consists
of two error amplifiers, a minimum circuit, a multiplier,
a transmission gate, a current comparator, a low output
current oscillator and a master latch, which will be ex-
plained in the following sections. The part also features a
sample-and-hold to sample the output voltage from the
third winding. A comparator is used to detect discontinu-
ous conduction mode (DCM) with a cap connected to the
third winding. The part features a 1.9A gate driver.
The LT3798 is designed for both off-line and DC applica-
tions. The EN/UVLO and a resistor divider can be configured
for a micropower hysteretic start-up. In the Block Diagram,
R3 is used to stand off the high voltage supply voltage.
The internal LDO starts to supply current to the INTV
CC
when V
IN
is above 2.5V. The V
IN
and INTV
CC
capacitors are
charged by the current from R3. When V
IN
exceeds the
turn-on threshold and INTV
CC
is in regulation at 10V, the
OPERATION
part begins to switch. The V
IN
hysteresis is set by the EN/
UVLO resistor divider. The third winding provides power
to V
IN
when its voltage is higher than the V
IN
voltage. A
voltage shunt is provided for fault protection and can sink
8mA of current when V
IN
is over 40V.
During a typical cycle, the gate driver turns the external
MOSFET on and a current flows through the primary wind-
ing. This current increases at a rate proportional to the
input voltage and inversely proportional to the magnetizing
inductance of the transformer. The control loop determines
the maximum current and the current comparator turns
the switch off when the current level is reached. When the
switch turns off, the energy in the core of the transformer
flows out the secondary winding through the output diode,
D1. This current decreases at a rate proportional to the
output voltage. When the current decreases to zero, the
output diode turns off and voltage across the secondary
winding starts to oscillate from the parasitic capacitance
and the magnetizing inductance of the transformer. Since
all windings have the same voltage across them, the third
winding rings too. The capacitor connected to the DCM
pin, C1, trips the comparator A2, which serves as a dv/dt
detector, when the ringing occurs. This timing information
is used to calculate the output current and will be described
below. The dv/dt detector waits for the ringing waveform
to reach its minimum value and then the switch turns back
on. This switching behavior is similar to zero volt switching
and minimizes the amount of energy lost when the switch
is turned back on and improves efficiency as much as
5%. Since this part operates on the edge of continuous
conduction mode and discontinuous conduction mode,
the operating mode is called critical conduction mode (or
boundary conduction mode).
Primary Side Control Loops
The LT3798 achieves constant current/constant voltage
operation by using two separate error amplifiers. These
two amplifiers are then fed to a circuit that outputs the
lower voltage of the two, shown as the "minimum" block in
the Block Diagram. This voltage is converted to a current
before being fed into the multiplier.
