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MC33680

Dual DC−DC Regulator for Electronic Organizer

ON Semiconductor 

MC33680

DETAILED OPERATING DESCRIPTION

General

The MC33680 is a dual DC−DC regulator designed for

electronic organizer applications. Both regulators apply

Pulse−Frequency−Modulation (PFM). The main boost

regulator output can be externally adjusted from 2.7V to 5V.

An internal synchronous rectifier is used to ensure high

efficiency (achieve 87%). The auxiliary regulator with a

built−in power transistor can be configured to produce a

wide range of positive voltage (can be used for LCD contrast

voltage). This voltage can be adjusted from +5V to +25V by

an external potentiometer.

The MC33680 has been designed for battery powered

hand−held products. With the low start−up voltage from 1V

and the low quiescent current (typical 35 μA), the MC33680

is best suited to operate from 1 to 2 AA/ AAA cell.

Moreover, supervisory functions such as low battery

detection, CPU Power−Good signal, and back−up battery

control, are also included in the chip. It makes the MC33680

the best one−chip power management solution for

applications such as electronic organizers and PDAs.

Pulse Frequency Modulation (PFM)

Both regulators apply PFM. With this switching scheme,

every cycle is started as the feedback voltage is lower than

the internal reference. This is normally performed by

internal comparator. As cycle starts, Low−Side switch (i.e.

M1 in Figure 1) is turned ON for a fixed ON time duration

(namely, Ton) unless current limit comparator senses coil

current has reached its preset limit. In the latter case, M1 is

OFF instantly. So Ton is defined as the maximum ON time

of M1. When M1 is ON, coil current ramps up, so energy is

being stored inside the coil. At the moment just after M1 is

OFF, the Synchronous Rectifier (i.e. M2 in Figure 1) or any

rectification device (such as Schottky Diode of Auxiliary

Regulator) is turned ON to direct coil current to charge up

the output bulk capacitor. Provided that coil current limit is

not reached, every switching cycle delivers fixed amount of

energy to the bulk capacitor. For higher loading, a larger

amount of energy (Charge) is withdrawn from the bulk

capacitor, and a larger amount of Charge is then supplied to

the bulk capacitor to regulate output voltage. This implies

switching frequency is increased; and vice−versa.

Main Regulator

Figure 18 shows the simplified block diagram of Main

Regulator. Notice that precise bias current Iref is generated

by a VI converter and external resistor RIref, where

Iref

+

0.5

RIref

(A)

This bias current is used for all internal current bias as well

as setting VMAIN value. For the latter application, Iref is

doubled and fed as current sink at Pin 1. With external

resistor RMAINb tied from Pin1 to Pin32, a constant voltage

level shift is generated in between the two pins. In

close−loop operation, voltage at Pin 1 (i.e. Output feedback

voltage) is needed to be regulated at the internal reference

voltage level, 1.22V. Therefore, the delta voltage across Pin

1 and Pin 32 which can be adjusted by RMAINb determines

the Main Output voltage. If the feedback voltage drops

below 1.22V, internal comparator sets switching cycle to

start. So, VMAIN can be calculated as follows.

VMAIN

+

1.22

)

RMAINb

RIref

(V)

From the above equation, although VMAIN can be

adjusted by RMAINb and RIref ratio, for setting VMAIN, it

is suggested, by changing RMAINb value with RIref kept at

480K. Since changing RIref will alter internal bias current

which will affect timing functions of Max ON time (TON1)

and Min OFF time (TOFF1). Their relationships are as

follows;

TON 1 + 1.7 10–11 RIref (S)

TOFF 1 + 6.4 10–12 RIref (S)

Continuous Conduction Mode and Discontinuous

Conduction Mode

In Figure 19, regulator is operating at Continuous

Conduction Mode. A switching cycle is started as the output

feedback voltage drops below internal voltage reference

VREF. At that instant, the coil current is not yet zero, and it

starts to ramp up for the next cycle. As the coil current ramps

up, loading makes the output voltage to decrease as the

energy supply path to the output bulk capacitor is

disconnected. After Ton elapses, M1 is OFF, M2 is ON,

energy is pumped to the bulk capacitor. Output voltage is

increased as excessive charge is pumped in, then it is

decreased after the coil current drops below the loading.

Notice the abrupt spike of output voltage is due to ESR of the

bulk capacitor. Feedback voltage can be resistor−divided

down or level−shift down from the output voltage. As this

feedback voltage drops below VREF, next switching cycle

starts.

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