LTC3569IFE-TRPBF 查看數據表（PDF） - Linear Technology

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LTC3569IFE-TRPBF

Triple Buck Regulator with 1.2A and Two 600mA Outputs and Individual Programmable References

Linear Technology 

LTC3569

applications information

Selecting The Inductors

Calculating the inductor values for 30% ripple current at

maximum SVIN:

L1 = 2.5V/(2.25MHz •240mA)•(1–2.5V/4.2V)= 1.9µH

L2 = 1.8V/(2.25MHz •120mA)•(1–1.8V/4.2V)= 3.8µH

L3 = 1.2V/(2.25MHz •120mA)•(1–1.2V/4.2V)= 3.1µH

Choosing a vendor’s closest values gives L1 = 2.2µH, L2

= L3 = 3.3µH. These values result in the maximum ripple

currents of:

ΔIL1 = 2.5V/(2.25MHz•2.2µH)•(1–2.5V/4.2V) = 204mA

ΔIL2 = 1.8V/(2.25MHz•3.3µH)•(1–1.8V/4.2V) = 139mA

ΔIL3 = 1.2V/(2.25MHz•3.3µH)•(1–1.2V/4.2V) = 115mA

Selecting The Output Capacitors

The value of the output capacitors are calculated based

on a 5% load droop for maximum load current step. The

output droop is usually about 2.5 times the linear drop

of the first cycle and is estimated based on the following

formula:

COUT = 2.5•IOUT(MAX)/(fCLK•VDROOP)

The output capacitor values are calculated as:

COUT1 = 2.5•800mA/(2.25MHz•125mV) = 7.1µF

COUT2 = 2.5•400mA/(2.25MHz•90mV) = 4.9µF

COUT3 = 2.5•400mA/(2.25MHz•60mV) = 7.4µF

Choosing the closest standard values gives, COUT1 = 10µF,

COUT2 = 4.7µF and COUT3 = 10µF.

A 22µF input capacitor is selected since the Li-Ion battery

has sufficiently low output impedance.

Setting The Output Voltages

Without toggling the EN pins the LTC3569 develops a 0.8V

reference voltage for each of the feedback pins. The output

voltages are set by a resistive divider as follows:

VOUT = 0.8•(1 + R1/R2)

The resistors in Figure 5 are selected as the nearest 1%

standard resistor values. To improve frequency response

feedforward capacitors of 10pF and 20pF are used.

VIN 2.9V TO 4.2V

22µF SVIN PVIN SW1

EN1

FB1

EN2

LTC3569

EN3

RT

SW2

MODE

470k

FB2

PGOOD

SW3

FB3

SGND PGND

L1

2.2µH*

243k

115k

L2

3.3µH**

187k

150k

L3

3.3µH**

187k

374k

* WURTH 7447745022

** WURTH 7447745033

10pF

20pF

20pF

OUT1

2.5V AT 800mA

COUT1

10µF

OUT2

1.8V AT 400mA

COUT2

4.7µF

OUT3

1.2V AT 400mA

COUT3

10µF

3569 F05a

Design Example 1: Burst Mode Operation

100

VIN = 2.9V TO 4.2V

90

80

70

60

BUCK1 = 2.5V

BUCK2 = 1.8V

BUCK3 = 1.2V

50

0.1

1

10 100 1000 10000

ILOAD (mA)

3569 F05b

Figure 5. Triple Buck DC/DC Regulators: 800mA, 400mA, 400mA

3569fe

18

For more information www.linear.com/LTC3569

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