CS5111 datasheet(7/10 Pages) CHERRY

7 / 10 page

Application Notes

Circuit Description: continued

This section outlines a procedure for designing a boost switch-

ing power supply operating in the discontinuous mode.

Step 1

Determine the output power required by the load.

POUT = IOUTVOUT

(1)

Step 2

Choose COSC based on the target oscillator frequency with an

external resistor value, RBIAS = 64.9kΩ. (See Figure 1d).

Figure 7: Voltage and current waveforms for boost topology in CS5111.

Step 3

Next select the output voltage feedback sense resistor

divider as follows (Figure 8).

For VFB1 active, choose a value for R1 and then solve for

REQ where:

REQ =

(3a)

For VFB2 active, find:

VFB1 = VOUT

, (3b)

and then calculate R2 where:

R2 =

. (3c)

Then find R3, where:

R3 = REQ - R2.

(3d)

VFB1 - VFB2

VFB1/REQ

VR2

IR2

)

REQ

R1 + REQ

(

VSW

VIN

VSAT

ISW

IPeak

VOUT

Design Procedure for Boost Topology

Figure 6: Block diagram of the 1.4A current mode control switching regulator portion of the CS5111 in a boost configuration.

VIN

VREG

1.4A

VSW

COMP

VFB1

VFB2

SELECT

VLIN

IBIAS

Over Voltage

COSC

Gnd

Bandgap

Reference

Oscillator

Switcher Shutdown

Switcher

Error

Amplifier

Current Sense Amplifier

ENABLE

Multiplexer

COMP

Base Drive

Logic

RBIAS

64.9kΩ

1.25V

VOUT

COUT

VOUT

VFB1

VFB2

VR2

REQ

{

Figure 8. Feedback sense

resistor divider connected

between VOUT and ground.

-1

VOUT

VFB1

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