SP6641B datasheet(7/13 Pages) EXAR

7 / 13 page

Rev. 5/26/05,

*Patent Pending

SP6641A/6641B 500mA Alkaline DC/DC Boost Regulator in SOT-23

The SP6641 feed forward off-time control de-

livers more load current than constant off-time

control because the input battery voltage drops

during its life cycle. The term (IPK – KOFF/2L) is

the average current delivered to the output ca-

pacitor during the discharge phase. This is con-

stant with respect to input and output voltage.

With constant off-time control, the average dis-

charge current term becomes

(IPK-TOFF*(VOUT-VIN)/2L),

which decreases as the input voltage drops.

Table 1 illustrates the average inductor current

delivered to the load during discharge versus the

input voltage. The SP6641 feed forward off-

time control and the constant off-time control

are compared. For purposes of illustration, the

off times of each control scheme are normalized

at a typical two cell alkaline input voltage of

2.6V. The values used in Table 1 are:

IPK = 0.33A

L = 22

µH

VOUT= 3.3V

TOFF (SP6641) = 1.5V*µs/(3.3-VIN)

TOFF (constant) = 2.14µs

SP6641A

Constant TOFF

VIN

TOFF

Avg IL

TOFF

Avg IL

3.0

5.00

µs

0.30A

2.14

µs

0.32A

2.6

2.14

µs

0.30A

2.14

µs

0.30A

2.0

1.15

µs

0.30A

2.14

µs

0.27A

1.3

0.75

µs

0.30A

2.14

µs

0.23A

1.0

0.65

µs

0.30A

2.14

µs

0.22A

Table 1- Average IL vs. Input Voltage

The following equation defines the burst mode

frequency under heavy load conditions:

where:

VD = Forward schottky drop, (0.4V, typ)

VC = Average charging switch drop,

Rnmos*IPK, typically 0.1V

Ignoring the conduction losses of VD and VC,

the burst frequency equation simplifies to:

FBURST =

(VOUT – VIN)VIN

KOFFVOUT

Startup

The internal regulator circuitry is bootstrapped

to the VOUT pin. This requires a low voltage

oscillator and charging switch powered from the

VBATT pin to pump up the output voltage until

the reference is established. The reference pro-

vides a REFREADY signal that determines when

output control is handed over to the regulator.

REFREADY shuts down the startup circuit and

enables the regulator when the reference is valid

and VOUT is above +1.9V. Once the regulator is

given control it will continue to pump up the

output at full power until regulation is reached.

For two cell alkaline input voltages and above,

the output voltage will be pulled above +1.9V

quickly through the rectifying diode before the

reference has a chance to establish. In this sce-

nario the startup circuit will coarsely regulate

around +2.8V until the REFREADY signal as-

serts. This keeps the output from overshooting

in startup with higher input voltages.

Startup is guaranteed at +0.9V at room tempera-

ture with a 3k

Ω load. Heavier loads will require

a higher input voltage.

Shutdown/Enable Control

Pin 4 of the device is a VBATT referred control

pin that shuts down the converter with the pin

tied to ground, or enables the converter with the

pin tied to VBATT. When the converter is shut-

down the power switch is opened and all circuit

biasing is extinguished leaving only junction leak-

age currents on supply pins 3 and 5. The output

voltage will droop to one diode drop below the

battery voltage through the rectifying diode.

After pin 4 is brought high, the startup circuit is

enabled and starts pumping up the output until

REFREADY hands over control to the internal

regulator.

OPERATION

Loop Regulation: continued

BURST =

(VOUT–VIN)( VIN–VC )

KOFF

VOUT + VD –VC

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