LT1683 datasheet(10/24 Pages) LINER

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LT1683

1683f

OPERATIO

In noise sensitive applications switching regulators tend

to be ruled out as a power supply option due to their

propensity for generating unwanted noise. When switch-

ing supplies are required due to efficiency or input/output

constraints, great pains must be taken to work around the

noise generated by a typical supply. These steps may

include pre and post regulator filtering, precise synchro-

nization of the power supply oscillator to an external clock,

synchronizing the rest of the circuit to the power supply

oscillator or halting power supply switching during noise

sensitive operations. The LT1683 greatly simplifies the

task of eliminating supply noise by enabling the design of

an inherently low noise switching regulator power supply.

The LT1683 is a fixed frequency, current mode switching

regulator with unique circuitry to control the voltage and

current slew rates of the output switches. Current mode

control provides excellent AC and DC line regulation and

simplifies loop compensation.

Slew control capability provides much greater control

over the power supply components that can create con-

ducted and radiated electromagnetic interference. Com-

pliance with EMI standards will be an easier task and will

require fewer external filtering components.

The LT1683 uses two external N-channel MOSFETs as the

power switches. This allows the user to tailor the drive

conditions to a wide range of voltages and currents.

CURRENT MODE CONTROL

Referring to the block diagram. A switching cycle begins

with an oscillator discharge pulse, which resets the RS

flip-flop, turning on one of the external MOSFET drivers.

The switch current is sensed across the external sense

resistor and the resulting voltage is amplified and com-

pared to the output of the error amplifier (VC pin). The

driver is turned off once the output of the current sense

amplifier exceeds the voltage on the VC pin. In this way

pulse by pulse current limit is achieved. The toggle flip-

flop ensures that the two MOSFETs are enabled on alter-

nate clock cycles. Internal slope compensation is provided

to ensure stability under high duty cycle conditions.

Output regulation is obtained using the error amp to set

the switch current trip point. The error amp is a

transconductance amplifier that integrates the difference

between the feedback output voltage and an internal 1.25V

reference. The output of the error amp adjusts the switch

current trip point to provide the required load current at the

desired regulated output voltage. This method of control-

ling current rather than voltage provides faster input

transient response, cycle-by-cycle current limiting for

better output switch protection and greater ease in com-

pensating the feedback loop. The VC pin is used for loop

compensation and current limit adjustment. During nor-

mal operation the VC voltage will be between 0.25V and

1.27V. An external clamp on VC or SS may be used for

lowering the current limit.

The negative voltage feedback amplifier allows for direct

regulation of negative output voltages. The voltage on the

NFB pin gets amplified by a gain of – 0.5 and driven on to

the FB input, i.e., the NFB pin regulates to –2.5V while the

amplifier output internally drives the FB pin to 1.25V as in

normal operation. The negative feedback amplifier input

impedance is 100k (typ) referred to ground.

Soft-Start

Control of the switch current during start up can be

obtained by using the SS pin. An external capacitor from

SS to ground is charged by an internal 9

µA current source.

The voltage on VC cannot exceed the voltage on SS. Thus

as the SS pin ramps up the VC voltage will be allowed to

ramp up. This will then provide for a smooth increase in

switch maximum current. SS will be discharged as a result

of the CS voltage exceeding the short circuit threshold of

approximately 0.22V.

Slew Control

Control of output voltage and current slew rates is achieved

via two feedback loops. One loop controls the MOSFET

drain dV/dt and the other loop controls the MOSFET dI/dt.

The voltage slew rate uses an external capacitor between

CAP A or CAP B and the respective MOSFET drain. These

integrating caps close the voltage feedback loop. The

external resistor RVSL sets the current for the integrator.

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