SS6642-27G 查看數據表（PDF） - Silicon Standard Corp.
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SS6642-27G Datasheet PDF : 22 Pages
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APPLICATION INFORMATION
GENERAL DESCRIPTION
The SS6642G PFM (pulse frequency modulation)
controller IC combines a switch-mode regulator, N-
channel power MOSFET, precision voltage refer-
ence, and voltage detector in a single monolithic
device. It offers extremely low quiescient 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 controllers transfer a discrete amount of en-
ergy per cycle and regulate the output voltage by
modulating the switching frequency with a constant
turn-on time. Switching frequency depends on load,
input voltage, and inductor value, and it can range
up to 100KHz. The SW on-resistance is typically
1.9 to 2.2Ω to minimize switch losses.
When the output voltage drops, the error comparator
enables the 100kHz oscillator that turns on the
MOSFET for around 7.5us and off for 2.5us. 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 the
current is low, thereby maintaining a steady voltage
across the load.
As the load increases, the output capacitor dis-
charges 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
of each cycle. Depending on the circuit, a PFM
controller can operate in either discontinuous mode
SS6642G
or continuous conduction mode. Continuous conduction
mode means that the inductor current does not
ramp down to zero during each cycle.
VIN
IIN
SW
EXT
Isw
ID
IOUT
+
VOUT
Ico
VEXT
IIN
IPK
ISW
Charge Co.
ID
IOUT
TDIS Discharge Co.
VSW
t
Discontinuous Conduction Mode
1/15/2005 Rev.2.10
www.SiliconStandard.com
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