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LT3798

3798f

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

VIN pin supplies power to an internal LDO that generates

10V at the INTVCC 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 INTVCC

when VIN is above 2.5V. The VIN and INTVCC capacitors are

charged by the current from R3. When VIN exceeds the

turn-on threshold and INTVCC is in regulation at 10V, the

OPERATION

part begins to switch. The VIN hysteresis is set by the EN/

UVLO resistor divider. The third winding provides power

to VIN when its voltage is higher than the VIN voltage. A

voltage shunt is provided for fault protection and can sink

8mA of current when VIN 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.

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