Datasheet LT1683 (Analog Devices) - 10

LT1683
OPERATION
In noise sensitive applications switching regulators tend Output regulation is obtained using the error amp to to be ruled out as a power supply option due to their pro- set the switch current trip point. The error amp is a pensity for generating unwanted noise. When switching transconductance amplifier that integrates the difference supplies are required due to efficiency or input/output between the feedback output voltage and an internal 1.25V constraints, great pains must be taken to work around reference. The output of the error amp adjusts the switch the noise generated by a typical supply. These steps may current trip point to provide the required load current include pre and post regulator filtering, precise synchroni- at the desired regulated output voltage. This method of zation of the power supply oscillator to an external clock, controlling current rather than voltage provides faster synchronizing the rest of the circuit to the power supply input transient response, cycle-by-cycle current limiting oscillator or halting power supply switching during noise for better output switch protection and greater ease in sensitive operations. The LT1683 greatly simplifies the task compensating the feedback loop. The VC pin is used for of eliminating supply noise by enabling the design of an loop compensation and current limit adjustment. During inherently low noise switching regulator power supply. normal operation the VC voltage will be between 0.25V The LT1683 is a fixed frequency, current mode switching and 1.27V. An external clamp on VC or SS may be used regulator with unique circuitry to control the voltage and for lowering the current limit. current slew rates of the output switches. Current mode The negative voltage feedback amplifier allows for direct control provides excellent AC and DC line regulation and regulation of negative output voltages. The voltage on the simplifies loop compensation. NFB pin gets amplified by a gain of – 0.5 and driven on to Slew control capability provides much greater control over the FB input, i.e., the NFB pin regulates to –2.5V while the the power supply components that can create conducted amplifier output internally drives the FB pin to 1.25V as in and radiated electromagnetic interference. Compliance normal operation. The negative feedback amplifier input with EMI standards will be an easier task and will require impedance is 100k (typ) referred to ground. fewer external filtering components.
Soft-Start
The LT1683 uses two external N-channel MOSFETs as the Control of the switch current during start-up can be power switches. This allows the user to tailor the drive obtained by using the SS pin. An external capacitor from conditions to a wide range of voltages and currents. SS to ground is charged by an internal 9µA current source. The voltage on VC cannot exceed the voltage on SS. Thus
CURRENT MODE CONTROL
as the SS pin ramps up the VC voltage will be allowed to Referring to the Block Diagram. A switching cycle begins ramp up. This will then provide for a smooth increase in with an oscillator discharge pulse, which resets the RS switch maximum current. SS will be discharged as a result flip-flop, turning on one of the external MOSFET drivers. of the CS voltage exceeding the short-circuit threshold of The switch current is sensed across the external sense approximately 0.22V. resistor and the resulting voltage is amplified and com- pared to the output of the error amplifier (V
Slew Control
C pin). The driver is turned off once the output of the current sense Control of output voltage and current slew rates is achieved amplifier exceeds the voltage on the VC pin. In this way via two feedback loops. One loop controls the MOSFET drain pulse by pulse current limit is achieved. The toggle flip-flop dV/dt and the other loop controls the MOSFET dI/dt. ensures that the two MOSFETs are enabled on alternate The voltage slew rate uses an external capacitor between clock cycles. Internal slope compensation is provided to CAP A or CAP B and the respective MOSFET drain. These ensure stability under high duty cycle conditions. integrating caps close the voltage feedback loop. The external resistor, RVSL, sets the current for the integrator. 1683fd 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Test Circuits Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts