DESCRIPCIÓN

The LT1776 is a wide input range, high efficiency Buck (step-down) switching regulator. The monolithic die includes all oscillator, control and protection circuitry. The part can accept input voltages as high as 60V and contains an output switch rated at 700mA peak current. Current mode control delivers excellent dynamic input supply rejection and short-circuit protection.

The LT1776 contains several features to enhance efficiency. The internal control circuitry is normally powered via the VCC pin, thereby minimizing power drawn directly from the VIN supply (see Applications Information). The action of the LT1776 switch circuitry is also load dependent.  At medium to high loads, the output switch circuitry maintains fast rise time for good efficiency.  At light loads, rise time is deliberately reduced to avoid pulse skipping behavior.

The available SO-8 package and 200kHz switching frequency allow for minimal PC board area requirements.

 

CARACTERÍSTICAS

Wide Input Range: 7.4V to 40V

Tolerates Input Transients to 60V

700mA Peak Switch Rating

Adaptive Switch Drive Maintains Efficiency at High Load Without Pulse Skipping at Light Load

True Current Mode Control

200kHz Fixed Operating Frequency

Synchronizable to 400kHz

Low Supply Current in Shutdown: 30mA

Available in 8-Pin SO and PDIP Packages

 

APLICACIONES

Automotive DC/DC Converters

Cellular Phone Battery Charger Accessories

IEEE 1394 Step-Down Converters

 

INFORMACIÓN SOBRE APLICACIONES

limits the inductor’s current carrying capability as the I2R power threatens to overheat the inductor. If applicable, remember to include the condition of output short circuit. Although the peak current rating of the inductor can be exceeded in short-circuit operation, as core saturation perse is not destructive to the core, excess resistive self

heating is still a potential problem.

The final inductor selection is generally based on cost,which usually translates into choosing the smallest physical size part that meets the desired inductance value,resistance and current carrying capability. An additional factor to consider is that of physical construction. Briefly

stated, “open” inductors built on a rod- or barrel-shaped core generally offer the smallest physical size and lowest cost. However their open construction does not contain the resulting magnetic field, and they may not be acceptable in RFI-sensitive applications. Toroidal style inductors, many available in surface mount configuration, offer improved RFI performance, generally at an increase in cost and physical size. And although custom design is always a possibility, most potential LT1776 applications can be handled by the array of standard, off-the-shelf inductor products offered by the major suppliers

 

Selecting Freewheeling Diode

Highest efficiency operation requires the use of a Schottky type diode. DC switching losses are minimized due to its low forward voltage drop, and AC behavior is benign due to its lack of a significant reverse recovery time. Schottky diodes are generally available with reverse voltage ratings of 60V and even 100V, and are price competitive with other types. The use of so-called “ultrafast” recovery diodes is generally not recommended. When operating in continuous mode, the reverse recovery time exhibited by “ultrafast” diodes will result in a slingshot type effect. The power internal switch will ramp up VIN current into the diode in an attempt to get it to recover. Then, when the diode has finally turned off, some tens of nanoseconds later, the VSW node voltage ramps up at an extremely high dV/dt, perhaps 5 to even 10V/ns! With real world lead inductances, the VSW node can easily overshoot the VIN rail. This canresult in poor RFI behavior and if the overshoot is severe enough, damage the IC itself.

 

Selecting Bypass Capacitors

The basic topology as shown in Figure 1 uses two bypass capacitors, one for the VIN input supply and one for the VOUT output supply. User selection of an appropriate output capacitor is rela tively easy, as this capacitor sees only the AC ripple current in the inductor. As the LT1776 is designed for buck or step-down applications, output voltage will nearly always be compatible with tantalum type capacitors, which are generally available in ratings up to 35V or so. These tantalum types offer good volumetric efficiency and many are available with specified ESR performance. The product of inductor AC ripple current and output capacitor ESR will manifest itself as peak-to-peak voltage ripple on the output node. (Note: If this ripple becomes too large, heavier control loop compensation, at least at the switching fre quency, may be required on the VC pin.) The most de manding applications, requiring very low output ripple, may be best served not with a single extremely large output capacitor, but instead by the common technique of a separate L/C lowpass post filter in series with the output. (In this case, “Two caps are better than one”.) The input bypass capacitor is normally a more difficult choice. In a typical application e.g., 40VIN to 5VOUT, relatively heavy VIN current is drawn by the power switch for only a small portion of the oscillator period (low ON duty cycle). The resulting RMS ripple current, for which the capacitor must be rated, is often several times the DC average VIN current. Similarly, the “glitch” seen on the VIN supply as the power switch turns on and off will be related to the product of capacitor ESR, and the relatively high instantaneous current drawn by the switch. To compound these problems is the fact that most of these applications will be designed for a relatively high input voltage, for which tantalum capacitors are generally unavailable. Rela tively bulky “high frequency” aluminum electrolytic types, specifically constructed and rated for switching supply applications, may be the only choice.