SP6648 Datasheet, PDF(11/16 Page) Sipex Corporation – Ultra-low Quiescent Current, High Efficiency Boost Regulator

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SP6648 Datasheet, PDF (11/16 Pages) Sipex Corporation – Ultra-low Quiescent Current, High Efficiency Boost Regulator

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APPLICATIONS INFORMATION : Continued

VOUT Programming

The SP6648 can be programmed as either a

voltage source or a current source. To program

the SP6648 as voltage source, the SP6648 re-

quires 2 feedback resistors R1 & R2, as shown

in the SP6648EB evaluation board schematic, to

control the output voltage. To set VOUT in the

voltage mode, use the equation:

R1 = [(VOUT/1.25)-1] * R2

Using the RLIM Function

The peak inductor current, IPEAK, is programmed

externally by the RLIM resistor connected be-

tween the RLIM pin and GND. The peak inductor

current is defined by:

IPEAK = 1600/RLIM

The saturation current specified for the inductor

needs to be greater than the peak current to avoid

saturating the inductor, which would result in a

loss in efficiency and could damage the induc-

tor. The SP6648 evaluation board uses a RLIM

value of 1.87K for an IPEAK = 850mA to allow

the circuit to deliver up to 180mA for 1.3V input

and 400mA for 2.6V input. Other values could

be selected using the above relationships.

Using the LBON - Low Battery Output

Function

The SP6648 will regulate the output until the

input battery is completely discharged or until

the under voltage lock-out (UVLO) occurs at

VBATT = 0.61V. To provide a low battery warn-

ing, the Low Battery Output function of the

SP6648 can be used. LBON is programmed

externally by the R3 and R4 resistor divider

connected between VBATT , the LBI input pin

and GND. The LBON is an open drain output,

which is active low and is pulled up by a 1M

resistor R5 to VOUT. When the LBI comparator

falling threshold of 0.61V is reached, the LBON

output goes low as determined by the relation-

ship:

VLOWBATT = 0.61 * [(R3 + R4)/R4]

The SP6648 evaluation board R3 & R4 resistors

have been set to trip for a falling battery thresh-

old of about 2.0V. Using this relationship, other

low battery threshold values can be set by the

user.

UVLO the Under Voltage Lock-Out Function

Once started up, the SP6648 will regulate the

output until the input battery is completely dis-

charged or until the under voltage lock-out

(UVLO) occurs at VBATT = 0.61V. The UVLO

function will completely open all switches until

the battery again rises above the 0.61V thresh-

old.

Maximum Startup Current

It should be noted that for low input voltages the

SP6648 startup circuit can not support large

load currents at startup. In startup the SP6648

needs to boost the output from zero volts using

the input voltage. Once the output is greater than

1.9V the operate circuit takes over and the

SP6648 can supply much more current. Curves

of maximum load current in startup for the

SP6648 are shown in the typical performance

characteristics and can be compared with the

page one curve for maximum load current in

operation.

For 1-cell battery applications, it is recommended

to apply any large load current after the SP6648

has started up, typically in a few milliseconds.

This is typically not a problem in many applica-

tions where the load is a processor whose load

current is low until the processor voltage comes

up.

Date: 7/19/04

SP6648 Ultra-low Quiescent Current, High Efficiency Boost Regulator

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