Datasheet LT1738 (Analog Devices) - 9

LT1738
U OPERATIO
In noise sensitive applications switching regulators tend transconductance amplifier that integrates the difference to be ruled out as a power supply option due to their between the feedback output voltage and an internal propensity for generating unwanted noise. When switch- 1.25V reference. The output of the error amp adjusts the ing supplies are required due to efficiency or input/output switch current trip point to provide the required load constraints, great pains must be taken to work around the current at the desired regulated output voltage. This noise generated by a typical supply. These steps may method of controlling current rather than voltage pro- include pre and post regulator filtering, precise synchro- vides faster input transient response, cycle-by-cycle nization of the power supply oscillator to an external clock, current limiting for better output switch protection and synchronizing the rest of the circuit to the power supply greater ease in compensating the feedback loop. The VC oscillator or halting power supply switching during noise pin is used for loop compensation and current limit sensitive operations. The LT1738 greatly simplifies the adjustment. During normal operation the VC voltage will task of eliminating supply noise by enabling the design of be between 0.25V and 1.27V. An external clamp on VC or an inherently low noise switching regulator power supply. SS may be used for lowering the current limit. The LT1738 is a fixed frequency, current mode switching The negative voltage feedback amplifier allows for direct regulator with unique circuitry to control the voltage and regulation of negative output voltages. The voltage on the current slew rates of the output switch. Current mode NFB pin gets amplified by a gain of – 0.5 and driven on to control provides excellent AC and DC line regulation and the FB input, i.e., the NFB pin regulates to –2.5V while the simplifies loop compensation. amplifier output internally drives the FB pin to 1.25V as in normal operation. The negative feedback amplifier input Slew control capability provides much greater control impedance is 100k (typ) referred to ground. over the power supply components that can create con- ducted and radiated electromagnetic interference. Com-
Soft-Start
pliance with EMI standards will be an easier task and will require fewer external filtering components. Control of the switch current during start up can be obtained by using the SS pin. An external capacitor from The LT1738 uses an external N-channel MOSFET as the SS to ground is charged by an internal 9µA current source. power switch. This allows the user to tailor the drive The voltage on V conditions to a wide range of voltages and currents. C 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
CURRENT MODE CONTROL
switch maximum current. SS will be discharged as a result Referring to the block diagram. A switching cycle begins of the CS voltage exceeding the short circuit threshold of with an oscillator discharge pulse, which resets the RS approximately 0.22V. flip-flop, turning on the GATE driver and the external
Slew Control
MOSFET. The switch current is sensed across the external sense resistor and the resulting voltage is amplified and Control of output voltage and current slew rates is achieved compared to the output of the error amplifier (VC pin). The via two feedback loops. One loop controls the MOSFET driver is turned off once the output of the current sense drain dV/dt and the other loop controls the MOSFET dI/dt. amplifier exceeds the voltage on the VC pin. In this way The voltage slew rate uses an external capacitor between pulse by pulse current limit is achieved. CAP and the MOSFET drain. This integrating cap closes the Internal slope compensation is provided to ensure stabil- voltage feedback loop. The external resistor RVSL sets the ity under high duty cycle conditions. current for the integrator. The voltage slew rate is thus inversely proportional to both the value of capacitor and Output regulation is obtained using the error amp to set R the switch current trip point. The error amp is a VSL. 1738fa 9