LTC1470 Просмотр технического описания (PDF) - Linear Technology

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LTC1470

Single and Dual PCMCIA Protected 3.3V/5V VCC Switches

Linear Technology 

LTC1470/LTC1471

APPLICATIONS INFORMATION

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

and applied to the input of the LT1313. (Note that the

auxiliary winding must be phased properly as shown in

Figure 3.)

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

enable lines, 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 subtracted

from the internal offset at the Sense– input (pin 14) of the

LTC1142HV. When this external offset cancels the built-in

25mV offset, Burst ModeTM operation is inhibited and the

LTC1142HV is forced into continuous mode operation. (See

LTC1142HV data sheet for further detail.) In this mode, the

15V auxiliary supply can be loaded without regard to the

loading on the 5V output of the LTC1142HV.

Continuous mode operation is only invoked when the LT1313

is programmed to 12V. If the LT1313 is programmed to 0V,

3.3V or 5V, power is obtained directly from the main power

source (battery pack) through diode D1. Again, the LT1313

output can be loaded without regard to the loading of the

main 5V output.

R4 and C4 absorb transient voltage spikes associated with

the leakage inductance inherent in T1’s secondary winding

and ensure that the auxiliary supply does not exceed 20V.

Auxiliary Power from the LTC1142 3.3V Output

For low-battery count applications (<6.5V) it is necessary to

modify the circuit of Figure 3. As the input voltage falls, the

5V duty cycle increases to the point where there is simply not

enough time to transfer energy from the 5V primary winding

to the auxiliary winding. For applications where 12V load

currents exist in conjunction with these low input voltages,

use the circuit shown in Figure 4. In this circuit, the auxiliary

15V supply is generated from an overwinding on the 3.3V

inductor of the LTC1142 regulator output.

In Figure 3, power is drawn directly from the batteries

through D1 when the regulator is in Burst Mode operation

and the VPP pins require 3.3V or 5V. In this circuit, however,

Q3 and Q4 force the LTC1142 3.3V regulator into continuous

mode operation whenever 3.3V, 5V or 12V is programmed

at the VPPOUT pins of the LT1313. (See the LT1312 and

LT1313 data sheets for further detail.)

Burst Mode is a trademark of Linear Technology Corporation.

1/2 LTC1142

(3.3V REG)

AENVPP0

AENVPP1

BENVPP0

BENVPP1

24

VIN

VIN

5.4V TO 11V

+

23

PDRIVE

Q1

C1

68µF

D2

MBRS1100

6

NDRIVE

SENSE+ 1

SENSE– 28

HC86

Q2

D3

MBRS130T3 T1*

3.37:1

R1

15µH

100Ω

C2

1000pF

R3

12k

R2

100Ω

Q3

2N7002

R4

0.033Ω

+ C3

220µF

15V AUX

SUPPLY

+

D4

C5

18V

68µF

3.3V

OUTPUT

HC86

Q4

2N7002

*CTX02-12753

COILTRONICS (407) 241-7876

1470/71 F04

Figure 4. Deriving 15V from the 3.3V Output of the LTC1142 for VPP Power

14701fa

9

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