4

Date: 6/7/06 Rev B

SP6648 Ultra-low Quiescent Current, High Efficiency Boost Regulator

© 2006 Sipex Corporation

THEORY OF OPERATION

Detailed Description

The SP6648 is a step-up DC-DC converter that

can start up with input voltages as low as 0.85V

(typically) and operates with an input voltage

down to 0.61V. Ultra low quiescent current of

12

μA provides excellent efficiency, up to 94%.

In addition to the main switch, a 0.3

Ω internal

MOSFET the SP6648 has an internal synchro-

nous rectifier, increasing efficiency and reduc-

ing the space requirements of an external diode.

An internal inductive-damping switch signifi-

cantly reduces inductive ringing for low-noise,

high efficiency operation. If the supply voltage

drops below 0.61V the SP6648 goes into under

voltage lock-out mode, thus opening both inter-

nal switches. An externally programmable low

battery detector with open drain output provides

the ability to flag a battery-low condition. The

inductor peak current is externally program-

mable to allow for a range of inductor values.

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 higher efficiency than tradi-

tional PWM converters are capable of deliver-

ing. At these loads, the switching frequency is

FUNCTIONAL DIAGRAM

SHDN

UVLO

0.61V

0.61V

SDI

IBIAS

SDI

INTERNAL

FB

LBI

Ipkset

current

control

current

reference

REF

FB

VOLOW

SQn

CHARGE

Q

R

charge end

IUC

QKILL

LX

undercurrent

comparator

INTERNAL

SUPPLY

Max

Ton

QKILL

PMOS

NMOS

LX

current

reference

overcurrent

comparator

SWITCH GROUND

GND

LBON

INTERNAL

GROUND

LOAD

Ref

Block

Min

switch

buffer

+

-

c

+

-

c

n

0.61V

SP6648

+

-

c

+

-

c

+

-

c

1.25V