4

Rev:001 Date:11/20/03

SP6649 Ultra-low Quiescent Current, High Efficiency Boost DC-DC Regulator

© Copyright 2002 Sipex Corporation

THEORY OF OPERATION

Detailed Desctiption:

The SP6649 is a step-up DC-DC converter that

starts up with input voltages as low as 0.85V

(typically) and operates with input voltages down

to 0.62V. The ultra low quiescent current of

12µA provides excellent efficiencies. In addi-

tion to the 0.3 internal MOSFET the SP6649

has an internal synchronous rectifier eliminat-

ing the need for an external diode. An internal

inductive-damping switch significantly reduces

inductive ringing. If the supply voltage drops

below 0.62V the SP6649 goes into under volt-

age lock-out opening up the internal switches.

An externally programmable low battery detec-

tor with open drain output provides the user the

ability to monitor the supply voltage. The induc-

tor peak current is externally programmable to

allow for a range of inductors.

Control Scheme:

A minimum off-time, current limited pulse fre-

quency modulation (PFM) control scheme com-

bines the high output power and efficiency of a

pulse width modulation (PWM) device with the

ultra low quiescent current of the traditional

PFM. At low to moderate output loads the PFM

control provides a higher efficiencies than tradi-

tional PWM converters are capable of deliver-

ing. At these loads the switching frequency is

BLOCK DIAGRAM

VBATT

SHDN

VBATT

VO

UVLO

0.61V

0.61V

SDI

IBIAS

SDI

INTERNAL

Vbatt

FB

LBI

Rlim

Ipkset

current

control

current

reference

REF

FB

VOLOW

SQn

CHARGE

Q

R

TOFF

charge end

IUC

QKILL

LX

undercurrent

comparator

INTERNAL

SUPPLY

VO

VBATT

Max

Ton

VOUT

QKILL

PMOS

NMOS

LX

current

reference

overcurrent

comparator

SWITCH GROUND

PGND

GND

LBON

INTERNAL

GROUND

LOAD

Ref

Block

Min

Toff

switch

buffer

+

-

c

+

-

c

n

0.61V

SP6649

+

-

c

+

-

c

+

-

c

1.25V