SP6642EU 查看數據表（PDF） - Signal Processing Technologies
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SP6642EU Datasheet PDF : 16 Pages
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DESCRIPTION
The SP6642/6643 devices are high-efficiency,
low-power step-up DC-DC converters ideal for
single alkaline cell applications such as pagers,
remote controls, and other low-power portable
end products.
The SP6642 features a 5nA logic-controlled
shutdown mode. The SP6643 features dedicated
low-battery detector circuitry. Both devices
contain a 0.8Ω synchronous rectifier, an
oscillator, a 0.6Ω N-channel MOSFET power
switch, an internal voltage reference, circuitry
for pulse-frequency-modulation, and an under
voltage comparator. The output voltage for the
SP6642/6643 devices can be adjusted from +2V
to +5.5V by manipulating two external resistors.
The output voltage is preset to +3.3V.
THEORY OF OPERATION
The SP6642/6643 devices are ideal for end
products that function with a single alkaline cell,
such as remote controls, pagers, and other portable
consumer products. Designers can implement
the SP6642/6643 devices into applications with
the following power management operating
states: 1. where the primary battery is good and
the load is active, and 2. where the primary
battery is good and the load is sleeping.
In the first operating state where the primary
supply is good and the load is active, the SP6642/
6643 devices typically offer 80% efficiency,
drawing tens of milliamps.
Applications will predominantly operate in the
second state where the primary supply is good
and the load is sleeping. The SP6642/6643
devices draw a very low quiescent current while
the load in its disabled state will draw typically
hundreds of microamps.
The pulse-frequency-modulation (PFM) circuitry
provides higher efficiencies at low to moderate
output loads than traditional PWM converters
are capable of delivering.
The on-time and minimum off-times are varied
as a function of the input and output voltages:
tON = K
VBATT
tOFF(MIN) = 1.2 x K
VOUT - VBATT
where tON is the on-time, K is the on-time constant
typically 25V-µs, VBATT is the supply voltage,
tOFF(MIN) is the minimum off-time, and VOUT is the
output voltage. This allows the SP6642/6643
devices to maintain a high efficiency over a
wide range of loads and input/output voltages.
The DC-DC converter is powered from V .
OUT
In a state where the error comparator detects that
the output voltage at V is too low, the internal
OUT
N-channel MOSFET switch is turned on until
the on-time is satisfied. Refer to Figures 20, 21,
22 and 23. During the on-time, current ramps up
in the inductor, storing energy in a magnetic
field. When the MOSFET turns off, during the
second half of each cycle the magnetic field
collapses. This causes the inductor voltage to
force current through the synchronous rectifier
transferring the stored energy from the inductor
to the output filter capacitor and the load. The
output filter capacitor stores charge while current
from the inductor is high and holds the output
voltage high until the second half of the next
switching cycle, smoothing power flow to the
load.
Internal Bootstrap Circuitry
The internal bootstrap circuitry contains a
low-voltage start-up oscillator that pumps up
the output voltage to approximately 1.9V so the
main DC-DC converter can function. At lower
battery supply voltages, the circuitry can start
up with low-load conditions. Designers can
reduce the load as needed to allow start-up with
input voltages below 1V. Refer to Figures 10 to
13. Once started, the output voltage can maintain
the load as the battery voltage decreases below
the initial start-up voltage. The start-up oscillator
is powered by VBATT driving a charge pump and
NMOS switch. During start-up, the P-channel
synchronous rectifier remains off and either its
body diode or an external diode is used as an
output rectifier.
Rev. 10-6-00
SP6642/6643 High Efficiency Step-Up DC-DC Converter
10
© Copyright 2000 Sipex Corporation
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