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Date: 7/20/05

SP7648 Ultra-low Quiescent Current, High Efficiency Boost Regulator

© Copyright 2005 Sipex Corporation

_________________________DETAILED

DESCRIPTION

The SP7648 is a step-up DC-DC converter

with an input voltage operation range from

2.7V to 4.7V. In addition to the main 0.3Ω

internal NMOSFET switch the SP7648 has

an internal synchronous rectifier, thereby

increasing efficiency and reducing the space

and cost of an external diode. An internal

inductive-damping switch significantly re-

duces inductive ringing for low noise-high

efficiency operation. If the supply voltage

drops below 0.61V the SP7648 goes into

under voltage lockout, thus opening both

internal switches. The inductor peak current

is externally programmable to allow for a

range of inductor values.

__________________________ CIRCUIT

LAYOUT

Printed circuit board layout is a critical part of

a power supply design. Poor designs can

result in excessive EMI on the feedback

paths and on the ground planes with applica-

tions involving high switching frequencies

and large peak currents. Excessive EMI can

result in instability or regulation errors. All

power components should be placed on the

PC board as closely as possible with the

traces kept short, direct, and wide (>50mils

or 1.25mm). Extra copper on the PC board

should be integrated into ground as a pseudo-

ground plane. On a multilayer PC board,

route the star ground using component-side

copper fill, then connect it to the internal

ground plane using vias. For the SP7648

THEORY OF OPERATION

devices, the inductor and input & output filter

capacitors should be soldered with their

ground pins as close together as possible in

must be bypassed directly to ground as

close to the SP7648 devices as possible

(within 0.2in or 5mm). The DC-DC converter

and any digital circuitry should be placed on

the opposite corner of the PC board as far

away from sensitive RF and analog input

stages. Noisy traces, such as from the LX

pin, should be kept away from the voltage-

using grounded copper to minimize EMI.

See the SP7648EB Evaluation Board

Manual for PC Board Layout design details.

________________ CONTROL SCHEME

A minimum off-time, current limited pulse

frequency modulation (PFM) control scheme

combines the high output power and effi-

ciency 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 traditional PWM converters

are capable of delivering. At these loads the

switching frequency is determined by a mini-