5

Mar16-06 Rev B

SP7648 Ultra-low Quiescent Current, High Efficiency Boost Regulator

© 2006 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 0.3

internal NMOSFET switch the SP7648 has

an internal synchronous rectifier, thereby

increasing efficiency and reducing the

space and cost of an external diode. An in-

ternal inductive-damping switch significantly

reduces 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 in-

ternal 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

resultinexcessiveEMIonthefeedbackpaths

and on the ground planes with applications

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

shouldbeintegratedintogroundasapseudo-

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

schemecombinesthehighoutputpowerand

efficiency of a pulse width modulation

(PWM) device with the ultra low quiescent

current of the traditional PFM. At low to mod-

erate output loads the PFM control provides

higher efficiency than traditional PWM con-

verters are capable of delivering. At these

loads the switching frequency is determined