SC1109ACSTR SEMTECH [Semtech Corporation], SC1109ACSTR Datasheet - Page 7

• Manufacturer Part Number: SC1109ACSTR
• Description: Synchronous PWM Controller with Dual Low Dropout Regulator Controllers
• Manufacturer: SEMTECH [Semtech Corporation]

An internal comparator with a 200mV reference moni-
tors the Drop across the upper FET, Once the Vdson of
the MOSFET exceeds the 200mV limit, the low side gate
is turned on and the upper FET is turned off. Also an
internal latch is set and the soft start capacitor is dis-
charged. Once the lower threshold of the soft start cir-
cuit is crossed, the same softstart sequence mentioned
previously is repeated. This sequence is repeated until
the over condition is removed.
The SC1109C utilizes an internal current source and an
external resistor connected from the OCSET pin to the
Mosfet’s supply to program a current limit level. This limit
is programmable by choosing the resistor according to
the level required. To reduce any noise pick up a 1nF
capacitor should be placed across the programing resis-
tor. The device operation is similar to the SC1109A/B
during the over current condition as mentioned above.
POWER MANAGEMENT
Application Information (Cont.)
2004 Semtech Corp.
Upper Gate
Lower Gate
Vtt Sho rted
PhaseNode
Lo wer Gate
Upper Gate
7
GATE DRIVERS
The Low side gate driver is supplied from VCC and provide
a peak source/sink current of and 500mA.
The high side gate drive is also capable of sourcing and
sinking peak currents of 500mA. The high side MOSFET
gate drive can be provided by an external 12V supply that
is connected from BST to GND. The actual gate to source
voltage of the upper MOSFET will approximately equal
7V (12V-VCC). If the external 12V supply is not available,
a classical boot strap technique can be implemented
from the VCC supply. A boot strap capacitor is connected
from BST to Phase while VCC is connected through a
diode (Schottky or other fast low VF diode) to the BST.
This will provide a gate to source voltage approximately
to VCC-Vdiode drop.
Shoot through control circuitry provides a 100ns dead time
to ensure both upper and lower MOSFET will not turn on
simultaneously and cause a shoot through condition.
DUAL LDO CONTROLLERS
SC1109 also provides two low drop out linear regulator
controllers that can be used to generate a 1.8V and a
1.5V output. The LDO output voltage is achieved by con-
trolling the voltage drop across an external MOSFET from
a 3.3V supply voltage.
The output voltage is sensed at the LDOS pin of the SC1109
and compared to an internal reference. The gate drive to
the external MOSFET is then adjusted until regulation is
achieved. In order to have sufficient voltage to the gate
drives of the external MOSFET, an internal charge pump is
utilized to boost the gate drive voltage to about two times
the VSTBY.
Low er Gat e
PhaseNode
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SC1109
Low er Gat e