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MC33680

Dual DC−DC Regulator for Electronic Organizer

ON Semiconductor 

MC33680

DETAILED OPERATING DESCRIPTION (Cont’d)

2 x Iref

1 VMAINFB

x2

0.5 V

Iref

IREF

8

RIref

480 kOhm

Voltage 1.22 V

Reference

VCOMP

COMP1

AGND

CMAINb

100 pF

RMAINb

1000 kOhm

VBAT

L1

33uH

31 VMAINSW

ZLC

COMP3

M2

+ve Edge Delay

for Max. ON Time

VDD

senseFET

RQ

M1

S Qb

DGND

1−SHOT

for Min. OFF Time

RQ

S

VDD

ILIM

DGND

COMP2

Voltage Reference

& Current Bias

Main Regulator

with Synchronous Rectifier

AGND

Figure 18. Simplified Block Diagram of Main Regulator

VMAIN

32 +

CMAIN

100 uF

VMAINGND

30

In Figure 20, regulator is operating at Discontinuous

Conduction Mode, waveforms are similar to those of Figure

19. However, coil current drops to zero before next

switching cycle starts.

To estimate conduction mode, below equation can be

used.

h

Iroom + 2

TON

L

Vin 2

Vout * ILOAD

where, η is efficiency, refer to Figure 4

if Iroom > 0, the regulator is at Discontinuous Conduction

mode

if Iroom = 0, the regulator is at Critical Conduction mode

where coil current just drops to zero and next cycle starts.

if Iroom < 0, the regulator is at Continuous Conduction

mode

For Continuous Conduction mode, provided that current

limit is not reached,

ǒ Ǔ TSW

+

TON

1*h

Vin

Vout

(S);

ǒ Ǔ Ipk

+

1

ILOAD

*

TON

TSW

Vin

)2

TON

L

(A)

For Discontinuous Conduction mode, provided that

current limit is not reached,

ǒ Ǔ TSW

+

Vin @ TO2N

2 @ L @ ILOAD @

Vout

h@Vin

*

1

(S);

Ipk

+

Vin

L

@

TON

(A)

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