PD70210AL datasheet(26/36 Pages) MICROCHIP

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Figure 5-1. Equivalent Diagrams for UVLO and Hysteresis

5.6

Setting the Voltage Divider for Output Rails

Generically, the equation is stated as:

Where, RUP is the name given to the upper resistor (connected to output rail) and RLOW is the name given to the

resistor connected to lower rail (usually IC ground). However, with so many topologies, in effect the following three

cases in all the typical schematics presented so far are present.

• Non-isolated topologies with simple divider connected directly to FB pin. For this, VX = 1.2V is used.

• Isolated topologies with divider to another reference (such as TL431 with an internal reference of 2.5V). For this,

VX = 2.5V is used.

• Non-isolated topologies with a differential divider connected to differential voltage amplifier of the LX7309. The

same preceeding divider equation is used, but with VX = 0.171V (that is, 1.2V divided by the gain of the

differential amplifier 7). Two identical dividers are required.

5.7

Selecting the Sense Resistor

In a Buck topology, the center of the switch current ramp equals the output current. To that, about 30% for the “IPEAK

+” peak current must be added because of the rising ramp caused by the inductor. That is a factor of 1.3. Some

headroom for proper transient response at maximum load must also be included. As the peak voltage on the sense

resistor is 0.2V, to leave headroom, it must be planned in such a way that the switch current peak stays at around

0.18V at the most, at maximum load. This means the following:

An adjust resistor must be placed in parallel (for example, the 22Ω placeholder).

For a Forward converter (Buck with a transformer), instead of the IOR load current as shown in the preceeding

equation, the reflected load current of IO/n can be used, where n is the turns ratio (number of primary-side turns

divided by number of secondary-side turns). The sense resistance must also be lowered further (by means of the

adjust resistor), to account for the magnetization current component on the switch side.

PD70211

Applications Information

Datasheet

DS00003672A-page 26

PD70210AL 数据表(PDF) 26 Page - Microchip Technology