Figure 1. MAXREFDES112# block diagram.

Detailed Circuit Description

The MAXREFDES112# reference design contains all the control circuitry and electric

components required for designing a 17V to 36V wide-input, 12V 10W output isolated

flyback converter power supply.

When the MOSFET is turned on, the current flows through the primary winding of the

transformer and stores the energy. The CS-pin input and the current sense resistor set the

peak current limit and provide overcurrent protection. When the MOSFET is tuned off, the

energy stored in the primary side is transferred to the secondary side.

The secondary side of the circuit consists of a diode, and a few output capacitors. When

the MOSFET on the primary side is off, the transformer pushes the current to the load at

the output and the remaining energy is stored in the output capacitors. When the MOSFET

is on, the diode prevents the current flow back into the transformer and discharges the

output capacitors.

The feedback network is typical of most isolated flyback converters. It is constructed using

a TL431 programmable shunt regulator, a 3000V isolation opto-coupler, and other RC

components.

Startup Voltage and Input Overvoltage Protection Setting (EN/UVLO, OVI)

The EN/UVLO pin in the MAX17596 serves as an enable/disable input, as well as an

accurate programmable undervoltage lockout (UVLO) pin. The MAX17596 does not begin

startup operations unless the EN/UVLO pin voltage exceeds 1.21V (typ). The MAX17596

turns off if the EN/UVLO pin voltage falls below 1.15V (typ). A resistor divider from the