6

Rev. 5/26/05,

*Patent Pending

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

© Copyright 2002 Sipex Corporation

2L

OPERATION

General Overview

The SP6641 is a high efficiency, low quiescent

current step-up DC-DC converter ideal for single

and dual cell alkaline and single cell Lithium Ion

battery applications such as medical monitors,

PDA’s, MP3 players, and other portable end

products. The SP6641’s 10

µA quiescent cur-

rent, low 0.3

Ω NFET switch, and unique PFM

control scheme combine to provide excellent

efficiency over a wide output power range.

Other features include a logic level enable con-

trol pin, guaranteed 0.9V startup, a tiny SOT23

5 pin package, and precise inductor peak current

control. SP6641A sources up to 90mA at 1.3V,

typ. and SP6641B sources up to 500mA at 2.6V,

typ. by supporting different peak inductor current

levels. Only two capacitors, an inductor, and a

diode are required to build a power supply for

the SP6641A. The SP6641B, 1A peak current

requires an additional small resistor and capaci-

Loop Regulation

The SP6641 combines a fixed inductor peak

current limit, a feed-forward minimum off-time

one-shot, and a precision loop comparator to

regulate the output voltage. Under light-load

conditions the loop operates as a standard PFM

converter. The frequency of fixed amplitude

inductor current triangles is modulated to regu-

late the load. Under heavy load conditions, the

converter adjusts the number of successive con-

tinuous mode current pulses to regulate the load.

Refer to the block diagram for the following

explanation of operating modes in loop regulation.

The output voltage is internally divided down and

fed to the negative terminal of the loop compara-

tor. A +1.25V bandgap reference voltage is ap-

plied to the positive terminal of the comparator. As

the output voltage droops below the regulation

threshold due to the load the loop comparator

“1”. This sets the SR latch and initiates inductor

charging by pulling the signal NGATE high. In-

ductor charging continues until the current reaches

the internally programmed limit, at which point,

the off-time one-shot is triggered.

opens the NMOS charge switch, and forces the

inductor to discharge through the rectifying

diode for a minimum time defined by the one-

shot duration. The end of the off-time pulse

releases the SR latch, and its output state is once

again determined by the output of the loop

ditions, the output voltage will have been pulled

above the regulation threshold during the mini-

logic “0”, and the NMOS charging switch will

remain open. The inductor current discharges

until it reaches zero or the loop comparator

triggers a new charge cycle.

Under a heavy load, the output voltage will

remain below the regulation point at the end of

has a logic value of 1 which immediately starts

a new charge/discharge cycle defined by the

peak inductor current and the minimum off-

time. The inductor current will remain in a

continuous conduction mode until the loop com-

parator indicates the output voltage is above the

regulation threshold, and the inductor current

will relax towards zero.

During continuous mode bursts, the inductor

current frequency and ripple amplitude are con-

trolled by the minimum off-time one-shot and

the input and output voltage levels. The SP6641

sets the minimum off-time to:

= Off-time Constant, typically 1.5

µs*V

= Output Voltage

= Input Voltage

mode equations yields the maximum output

current in regulation:

where:

η

= Efficiency, typically 0.80 to 0.90

= Programmed inductor peak current, typi-

cally 0.33A for the SP6641A, typically

1.0A for the SP6641B.

L= Inductor value