LTC1872BES6 查看數據表（PDF） - Linear Technology

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LTC1872BES6

Constant Frequency Current Mode Step-Up DC/DC Controller in ThinSOT

Linear Technology 

LTC1872B

APPLICATIONS INFORMATION

∆VOUT

≈

IO

•

VOUT +

VIN

VD

+

IRIPPLE

2





•

1



ESR2



+





2πf

1

C OUT





2









2

where f is the operating frequency, COUT is the output

capacitance and IRIPPLE is the ripple current in the induc-

tor.

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest ESR (size)

product of any aluminum electrolytic at a somewhat

higher price. The output capacitor RMS current is approxi-

mately equal to:

IPK • DC − DC2

where IPK is the peak inductor current and DC is the switch

duty cycle.

When using electrolytic output capacitors, if the ripple and

ESR requirements are met, there is likely to be far more

capacitance than required.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the ESR or RMS current

handling requirements of the application. Aluminum elec-

trolytic and dry tantalum capacitors are both available in

surface mount configurations. An excellent choice of

tantalum capacitors is the AVX TPS and KEMET T510

series of surface mount tantalum capacitors. Also,

ceramic capacitors in X5R pr X7R dielectrics offer excel-

lent performance.

Low Supply Operation

Although the LTC1872B can function down to approxi-

mately 2.0V, the maximum allowable output current is

reduced when VIN decreases below 3V. Figure 4 shows the

amount of change as the supply is reduced down to 2V.

Also shown in Figure 4 is the effect of VIN on VREF as VIN

goes below 2.3V.

Setting Output Voltage

The LTC1872B develops a 0.8V reference voltage between

the feedback (Pin 3) terminal and ground (see Figure 5). By

selecting resistor R1, a constant current is caused to flow

through R1 and R2 to set the overall output voltage. The

regulated output voltage is determined by:

VOUT = 0.8V1+ RR21

For most applications, an 80k resistor is suggested for R1.

To prevent stray pickup, locate resistors R1 and R2 close

to LTC1872B.

105

VREF

100

95

VITH

90

85

80

75

2.0 2.2

2.4

2.6

2.8

3.0

INPUT VOLTAGE (V)

1872B F04

Figure 4. Line Regulation of VREF and VITH

LTC1872B

3

VFB

VOUT

R2

R1

1872B F05

Figure 5. Setting Output Voltage

8

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