LTC1174 查看數據表（PDF） - Linear Technology

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LTC1174

High Efficiency Step-Down and Inverting DC/DC Converter

Linear Technology 

LTC1174

LTC1174-3.3/LTC1174-5

APPLICATIO S I FOR ATIO

LTC1174 VFB 1

6.8nF**

VOUT

R2

100pF*

R1

* ADJUSTABLE APPLICATIONS

** LOW NOISE APPLICATIONS

1174 F04

Figure 4. LTC1174 Adjustable Configuration

INPUT VOLTAGE

4V TO 12V

6

3

LBIN

VIN

SHUTDOWN 8

2

LBOUT

VOUT 1

7 IPGM

SW 5

LTC1174HV-5

GND

4

* AVX TPSD476K016

** COILTRONICS CTX50-4

+

0.1µF

47µF*

16V

×2

50µH** +

MBRS140T3

47µF*

16V

×2

VOUT

– 5V

45mA

1174 F05

Figure 5. Positive-to-Negative 5V Converter

output voltage is now taken off the GND pin. Therefore,

the maximum input voltage is now determined by the

difference between the absolute maximum voltage rating

and the output voltage. A maximum of 12V is specified in

Figure 5, giving the circuit a 1.5V of headroom for VIN. Note

that the circuit can operate from a minimum of 4V, making

it ideal for a 4 NiCad cell application. For a higher output

current circuit, please refer to the Typical Applications

section.

LTC1174-5 regulator and also to one or more loads in

parallel with the the regulator’s VIN. If the battery is dis-

connected while the LTC1174/LTC1174-3.3/LTC1174-5

regulator is supplying a light load and one of the parallel

circuits is a heavy load, the input capacitor of the LTC1174/

LTC1174-3.3/LTC1174-5 regulator could be pulled down

faster than the output capacitor, causing the absolute

maximum ratings to be exceeded. The result is often a

latchup which can be destructive if VIN is reapplied. Bat-

tery disconnect is possible as a result of mechanical stress,

bad battery contacts or use of a lithium-ion battery with

a built-in internal disconnect. The user needs to assess

his/her application to determine whether this situation

could occur. If so, additional protection is necessary.

Prevention against latchup can be accomplished by sim-

ply connecting a Schottky diode across the SW and VIN

pins as shown in Figure 6. The diode will normally be

reverse biased unless VIN is pulled below VOUT at which

time the diode will clamp the (VOUT – VIN) potential to less

than the 0.6V required for latchup. Note that a low leakage

Schottky should be used to minimize the effect on no-load

supply current. Schottky diodes such as MBR0530, BAS85

and BAT84 work well. Another more serious effect of the

protection diode leakage is that at no load with nothing to

provide a sink for this leakage current, the output voltage

can potentially float above the maximum allowable toler-

ance. To prevent this from occuring, a resistor must be

connected between VOUT and ground with a value low

enough to sink the maximum possible leakage current.

LATCHUP

PROTECTION

SCHOTTKY

Absolute Maximum Ratings and Latchup Prevention

The absolute maximum ratings specify that SW (Pin 5) can

never exceed VIN (Pin 6) by more than 0.3V. Normally this

situation should never occur. It could, however, if the

output is held up while the supply is pulled down. A con-

dition where this could potentially occur is when a battery

is supplying power to an LTC1174/LTC1174-3.3/

VIN

SW

LTC1174

LTC1174-3.3

LTC1174-5

VOUT

+

1174 F06

Figure 6. Preventing Absolute Maximum

Ratings from Being Exceeded

1174fe

10

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