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 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC1109

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.

Upper Gate

Lower Gate

PhaseNode

Vtt Sho rted

Upper Gate

Lower Gate

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.

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.

Low erGat e

PhaseNode

Low erGat e

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.

Application Information (Cont.)