Datasheet LT1533 (Analog Devices) - 7

LT1533
U OPERATIO
Output regulation is obtained using the error amp to set RCSL. The two control loops are combined internally to the switch current trip point. The error amp is a transcon- provide a smooth transition from current slew control to ductance amplifier that integrates the difference between voltage slew control. the feedback output voltage and an internal 1.25V refer- ence. The output of the error amp adjusts the switch
Internal Regulator
current trip point to provide the required load current at Most of the control circuitry operates from an internal 2.4V the desired regulated output voltage. This method of low dropout regulator that is powered from VIN. The controlling current rather than voltage provides faster internal low dropout design allows VIN to vary from 2.7V input transient response, cycle by cycle current limiting to 23V with virtually no change in device performance. for better output switch protection and greater ease in When the part is put into shutdown, the internal regulator compensating the feedback loop. is turned off, leaving only a small (12µA typ) current drain The V from V C pin serves three different purposes. It is used for IN. loop compensation, current limit adjustment and soft starting. During normal operation the V
Protection Features
C voltage will be between 0.2V and 1.33V. An external clamp may be used There are three modes of protection in the LT1533. The for lowering the current limit. A capacitor coupled to an first is overcurrent limit. This is achieved via the clamping external clamp can be used for soft starting. action of the VC pin. The second is thermal shutdown that The negative voltage feedback amplifier allows for direct disables both output drivers and pulls the VC pin low in the regulation of negative output voltages. The voltage on the event of excessive chip temperature. The third is under- NFB pin gets amplified by a gain of – 0.5 and driven onto voltage lockout that also disables both outputs the FB input, i.e., the NFB pin regulates to – 2.5V while the and pulls the VC pin low whenever VIN drops below 2.5V. amplifier output internally drives the FB pin to 1.25V as in
50% Duty Cycle Mode
normal operation. The negative feedback amplifier input impedance is 100k (typ) referred to ground. Since the LT1533 has dual out-of-phase outputs, it is ideal for driving push-pull transformers. For simple DC trans-
Slew Control
former applications, the part can be forced into a 50% duty Control of output voltage and current slew rates is done via cycle mode using the DUTY pin. Grounding the DUTY pin two feedback loops. One loop controls the output switch will override the internal control circuitry and force the collector voltage dV/dt and the other loop controls the outputs to switch with a 50% duty cycle at one-half the emitter current dI/dt. Output slew control is achieved by oscillator frequency. Slew control also applies in the 50% comparing the currents generated by these two slewing duty cycle mode. events to currents created by external resistors RVSL and
U U W U APPLICATIONS INFORMATION
Reducing EMI from switching power supplies has tradi- loads: switch voltage slew rate and switch current slew tionally invoked fear in designers. Many switchers are rate. The use of this part will reduce noise and EMI over designed solely on efficiency and as such produce wave- conventional switch mode controllers. Because these forms filled with high frequency harmonics that then variables are under control, a supply built with this part will propagate through the rest of the power supply. exhibit far less tendency to create EMI and less chance of wandering into problems during production. The LT1533 provides control over two of the more impor- tant variables for controlling EMI with switching inductive 7