AN2872 datasheet(10/23 Pages) STMICROELECTRONICS

10 / 23 page

Description

AN2872

10/23

Doc ID 15305 Rev 1

a MOSFET integrated in the VIPer16. The low side switch of the buck converter is

freewheeling diode D5 (800 V / 1 A). The energy is saved in inductor L2 and output capacitor

C5 during the ON time, when the MOSFET is conducting. The ON time (and consequently

also the duty cycle) is very short (in the range several %) due to the high ratio between the

input and the output voltage. During the OFF time, the MOSFET is switched OFF and diode

D5 conducts. Inductor L2 is discharged to the output and to capacitors C5 and C8 through

diodes D2 and D5. As the inductor L2 is, during OFF time, connected to C8 and C5 the

voltage drop over C8 (used for feedback regulation) is similar to the output voltage level.

Diode D1 is also applied, though not mandatory for correct operation, to reduce the power

consumption of the VIPer16 and consequently the standby power.

The VIPer16 operates in current mode. The FB pin is connected to the input of the error

amplifier, the output of the error amplifier is visible on the COMP pin and is connected to the

input of the built-in comparator for comparing the value from the output of the error amplifier

to the drain current. The voltage level expected on the FB pin is 3.3 V. It is possible to see

a rise in the output voltage at low or almost no load for this type of buck converter. This

effect is caused by the capacitor C8 discharging faster than C5 at light or no load, and C5,

moreover, is charged at every ON time. Consequently, the voltage over C8 is constant while

the voltage over C5 rises. To protect the output capacitors and the load against overvoltage

at light load, Zener diode D6 is included, limiting the output voltage to 15 V. It is possible to

use a simple resistive load instead of the Zener diode. The value of the minimum load

depends on the complete configuration of the components. If linear regulator U2 is not used,

the value of the resistor is 12 k

Ω (1 mA load), which could reduce the output voltage to

a similar level as the Zener diode.

Regarding current limitation, other features of VIPer16 can be activated by assembling

resistor R7, but it is not originally assembled to allow the delivery of maximum power.

A simple linear regulator is also assembled, generating 5 V on the output as an option to

complete the basic idea of generating 12 V and 5 V outputs. It should be highlighted that the

VIPer16 can also directly generate a 5V output (instead of 12 V). The 5 V can be set by

changing the value of resistor R3 to 4.7 kΩ.

AN2872 Arkusz danych(PDF) 10 Page - STMicroelectronics