CB2012T2R2M datasheet(14/16 Pages) LINEAR

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LTC3547

3547fa

APPLICATIO S I FOR ATIO

Design Example

As a design example, consider using the LTC3547 in a

portable application with a Li-Ion battery. The battery

provides a VIN ranging from 2.8V to 4.2V. The load on

each channel requires a maximum of 300mA in active

mode and 2mA in standby mode. The output voltages are

VOUT1 = 2.5V and VOUT2 = 1.8V.

Start with channel 1. First, calculate the inductor value

for about 40% ripple current (120mA in this example) at

maximum VIN. Using a derivation of Equation 1:

MHz

2 25

120

=−

⎛

⎝⎜

⎞

⎠⎟

.• (

)

•

µH

For the inductor, use the closest standard value of 4.7µH.

A 4.7µF capacitor should be more than sufﬁcient for this

output capacitor. As for the input capacitor, a typical value

of CIN = 4.7µF should sufﬁce, as the source impedance of

a Li-Ion battery is very low.

The feedback resistors program the output voltage. To

maintain high efﬁciency at light loads, the current in these

resistors should be kept small. Choosing 2µA with the

0.6V feedback voltage makes R1~300k. A close standard

1% resistor is 280k. Using Equation 4:

OUT

1 887

=−

⎛

⎝⎜

⎞

⎠⎟

•

An optional 10pF feedback capacity (CF1) may be used to

improve transient response.

Using the same analysis for channel 2 (VOUT2 = 1.8V),

the results are:

L2 = 3.81µH

R3 = 280k

R4 = 560k

Figure 4 shows the complete schematic for this example,

along with the efﬁciency curve and transient response.

Figure 4a. Design Example Circuit

Figure 4b. Efﬁciency vs Output Current

VIN

RUN2

RUN1

LTC3547

VFB2

SW2

SW1

VFB1

CF2, 10pF

CF1, 10pF

GND

VIN

2.5V TO 5.5V

VOUT2

1.8V AT 300mA

VOUT1

2.5V AT 300mA

3547 F04a

280k

562k

4.7

µH

4.7

µH

887k

COUT2

4.7

µF

4.7

µF

COUT1

4.7

µF

C1, C2, C3: TAIYO YUDEN JMK316BJ475ML

L1, L2: MURATA LQH32CN4R7M33

OUTPUT CURRENT (mA)

100

0.1

100

1000

3547 F04b

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

VOUT = 2.5V

OUTPUT CURRENT (mA)

100

0.1

100

1000

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

VOUT = 1.8V

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