Datasheet LT1534 (Analog Devices) - 8

LT1534/LT1534-1
U OPERATIO
The negative feedback amplifier allows for direct regula-
Internal Regulator
tion of negative output voltages. The voltage on the NFB Most of the control circuitry operates from an internal 2.4V pin gets amplified by a gain of – 0.5 and driven onto the FB low dropout regulator that is powered from V input, i.e., the NFB pin regulates to – 2.5V while the IN. The internal low dropout design allows V amplifier output internally drives the FB pin to 1.25V as in IN to vary from 2.7V to 23V with virtually no change in device performance. normal operation. The negative feedback amplifier input When the part is put into shutdown, the internal regulator impedance is 100k (typ) referred to ground. is turned off, leaving only a small (12µA typ) current drain from V
Slew Control
IN. Control of output voltage and current slew rates is done via
Protection Features
two feedback loops. One loop controls the output switch There are three modes of protection in the LT1534. The collector voltage dV/dt and the other loop controls the first is overcurrent limit. This is achieved via the clamping emitter current dI/dt. Output slew control is achieved by action of the V comparing the currents generated by these two slewing C pin. The second is thermal shutdown that disables both output drivers and pulls the V events to currents created by external resistors R C pin low in the VSL and event of excessive chip temperature. The third is under- RCSL. The two control loops are combined internally to voltage lockout that also disables both outputs provide a smooth transition from current slew control to and pulls the V voltage slew control. C pin low whenever VIN drops below 2.5V.
U U W U APPLICATIONS INFORMATION
Reducing EMI from switching power supplies has tradi- sawtooth design. A current defined by external resistor RT tionally invoked fear in designers. Many switchers are is used to charge and discharge the capacitor CT. The designed solely on efficiency and as such produce wave- discharge rate is approximately ten times the charge rate. forms filled with high frequency harmonics that then By allowing the user to have control over both compo- propagate through the rest of the power supply. nents, trimming of oscillator frequency can be more easily The LT1534 provides control over two of the more impor- achieved. tant variables for controlling EMI with switching inductive The external capacitance C loads: switch voltage slew rate and switch current slew T is chosen by: rate. The use of this part will reduce noise and EMI over CT(nF) = 2180/[fOSC(kHz) • RT(kΩ)] conventional switch mode controllers. Because these where fOSC is the desired oscillator frequency in kHz. variables are under control, a supply built with this part will exhibit far less tendency to create EMI and less chance of For RT equal to 16.9k, this simplifies to: wandering into problems during production. CT(nF) = 129/fOSC(kHz) It is beyond the scope of this data sheet to get into EMI (e.g., CT = 1.29nF for fOSC = 100kHz) fundamentals. AN70 contains much information concern- A good quality temperature stable capacitor should be ing noise in switching regulators and should be consulted. chosen.
Oscillator Frequency
Nominally RT should be 16.9k. Since it sets up current, its The oscillator determines the switching frequency and temperature coefficient should be selected to compliment therefore the fundamental positioning of all harmonics. the capacitor. Ideally, both should have low temperature The use of good quality external components is important coefficients. to ensure oscillator frequency stability. The oscillator is a 8