6
Rev. 5/26/05,
*Patent Pending
SP6641A/6641B 500mA Alkaline DC/DC Boost Regulator in SOT-23 © Copyright 2002 Sipex Corporation
I
OUT(MAX)
≈ η
(
V
IN
) (
I
PK
–
K
OFF
)
V
OUT
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 10µA quiescent cur-
rent, low 0.3Ω 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-
tor as a low pass filter for the V
OUT
IC power pin.
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
output (signal V
OUT(LOW)
) transitions to a logic
“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.
The off-time one-shot via signal T
OFF
resets the
SR latch regardless of the SET state (V
OUT(LOW)
),
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
comparator (V
OUT(LOW)
). Under light load con-
ditions, the output voltage will have been pulled
above the regulation threshold during the mini-
mum off-time, the signal V
OUT(LOW)
will be a
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
the off-time pulse. In this condition, V
OUT(LOW)
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:
T
OFF
=
K
OFF
(V
OUT
– V
IN
), where:
K
OFF
= Off-time Constant, typically 1.5µs*V
V
OUT
= Output Voltage
V
IN
= Input Voltage
Plugging the T
OFF
expression into the boost
mode equations yields the maximum output
current in regulation:
where:
η = Efficiency, typically 0.80 to 0.90
I
PK
= Programmed inductor peak current, typi-
cally 0.33A for the SP6641A, typically
1.0A for the SP6641B.
L= Inductor value
