SS6638-27G 查看數據表（PDF） - Silicon Standard Corp.
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SS6638-27G Datasheet PDF : 19 Pages
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APPLICATION INFORMATION
GENERAL DESCRIPTION
The SS6638G PFM (pulse frequency modulation)
converter IC combines a switch mode converter,
N-channel power MOSFET, precision voltage
reference, and voltage detector in a single
monolithic device. It offers both extreme low
quiescent current, high efficiency, and very low
gate threshold voltage to ensure start-up with low
battery voltage (0.8V typ.). Designed to maximize
battery life in portable products, it minimizes
switching losses by only switching as needed
to service the load.
PFM converters transfer a discrete amount of
energy per cycle and regulate the output voltage
by modulating the switching frequency with a
constant pulse width. Switching frequency
depends on load, input voltage, and inductor
value, and it can range up to 100kHz. The SW
on-resistance is typically 1 to 1.5Ω to minimize
switching losses.
When the output voltage drops, the error
comparator enables the 100KHz oscillator that turns
the MOSFET on for about 7.5µs and off for 2.5µs.
Turning on the MOSFET allows inductor current to
ramp up, storing energy in a magnetic field. When
The MOSFET turns off, inductor current is forced
through the diode to the output capacitor and load. As
the stored energy is depleted, the current ramps
down until the diode turns off. At this point,
the inductor may ring due to residual energy and stray
capacitance. The output capacitor stores charge when
the current flowing through the diode is high, and
releases it when current is low, thereby maintaining
a steady voltage across the load.
As the load increases, the output capacitor
discharges faster and the error comparator initiates
cycles sooner, increasing the switching frequency.
The maximum duty cycle ensures adequate time for
energy transfer to the output during the second half
SS6638G
of each cycle. Depending on the circuit, PFM
converters operate in either discontinuous mode
or continuous conduction mode. Continuous
conduction mode means that the inductor current
does not ramp to zero during each cycle.
VIN
IIN
SW
EXT
Isw
SS6638G
ID
IOUT
+
VOUT
Ico
VEXT
IIN
IPK
ISW
Charge Co.
ID
IOUT
TDIS Discharge Co.
VSW
t
Discontinuous Conduction Mode
8/21/2005 Rev.2.3
www.SiliconStandard.com
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