6

LT1312

Voltage Control Logic: The LT1312 has five possible

output modes: 0V, 3.3V, 5V, 12V and Hi-Z. These five

Sense input as described by the Truth Table.

the LT1312 automatically adjusts the regulated VPP out-

put voltage to 3.3V or 5V depending upon the voltage

voltage for the comparator is set at 4V and there is

approximately 50mV of hysteresis provided to ensure

clean switching between 3.3V and 5V.

VPP VALID Comparator: A voltage comparator monitors

the output voltage when the 12V mode is selected and is

driven low when the output is in regulation above 11V.

The LT1312 is a programmable output voltage, low-

dropout linear regulator designed specifically for PCMCIA

VPP drive applications. Input power is typically obtained

from a loosely regulated input supply between 13V and

20V (see Applications Information section for more detail

on the input power supply). The LT1312 consists of the

following blocks:

Low Dropout Voltage Linear Regulator: The heart of the

LT1312 is a PNP-based low-dropout voltage regulator

which drops the unregulated supply voltage from 13V to

20V down to 12V, 5V, 3.3V, 0V or Hi-Z depending upon the

regulator has built-in current limiting and thermal shut-

down to protect the device, the load, and the socket

against inadvertent short circuiting to ground.

The LT1312 is a voltage programmable linear regulator

designed specifically for PCMCIA VPP driver applications.

The device operates with very low quiescent current

(30

µA) in the 0V and Hi-Z modes of operation. In the Hi-Z

mode, the output leakage current falls to 1

µA. Unloaded

quiescent current rises to only 55

µA and 75µA when

programmed to 3.3V and 5V respectively. In addition to

the low quiescent currents, the LT1312 incorporates sev-

eral protection features which make it ideal for PCMCIA

applications. The LT1312 has built-in current limiting

(330mA) and thermal shutdown to protect the device and

the socket VPP pins against inadvertent short-circuit

conditions.

AUXILIARY WINDING POWER SUPPLIES

Because the LT1312 provides excellent output regulation,

the input power supply may be loosely regulated. One

convenient (and economic) source of power is an auxiliary

winding on the main 5V switching regulator inductor in the

main system power supply.

LTC

Figure 1 is a schematic diagram which describes how a

loosely regulated 14V power supply is created by adding

Burst Mode is a trademark of Linear Technology Corporation.

an auxiliary winding to the 5V inductor in a split 3.3V/5V

LTC1142HV power supply system. A turns ratio of 1:1.8 is

used for transformer T1 to ensure that the input voltage to

the LT1312 falls between 13V and 20V under all load

conditions. The 9V output from this additional winding is

rectified by diode D2, added to the main 5V output and

applied to the input of the LT1312. (Note that the auxiliary

winding must be phased properly as shown in Figure 1.)

The auxiliary winding is referenced to the 5V output which

provides DC current feedback from the auxiliary supply to

the main 5V section. The AC transient response is im-

proved by returning the negative lead of C5 to the 5V

output as shown.

When the 12V output is activated by a TTL high on the

Enable line, the 5V section of the LTC1142HV is forced into

continuous mode operation. A resistor divider composed

of R2, R3 and switch Q3 forces an offset which is sub-

of the LTC1142HV. When this external offset cancels the

built-in 25mV offset, Burst Mode

and the LTC1142HV is forced into continuous mode

operation. (See the LTC1142HV data sheet for further

detail). In this mode, the 14V auxiliary supply can be