LTC3607 datasheet(13/20 Pages) LINER

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LTC3607

3607fb

For more information www.linear.com/LTC3607

As an example, consider the case when the LTC3607 is in

dropout on both channels at an input voltage of 5V with

a load current of 600mA and an ambient temperature

of 25°C. From the Typical Performance Characteristics

graph of Switch Resistance, the RDS(ON) resistance of

the main switch is 0.9Ω. Therefore, power dissipated by

each channel is:

PD = I2 • RDS(ON) = 324mW

Running the two regulator channels under the same con-

ditions will result in a total power dissipation of 0.648W.

TheMSEpackagejunction-to-ambientthermalresistance,

θJA, is 37°C/W. Therefore, the junction temperature of

the regulator operating in a 25°C ambient temperature is

approximately:

TJ = 0.648W • 37°C/W + 25°C = 49°C

Design Example

As a design example, consider using the LTC3607 in a

portable application with a dual lithium-ion battery. The

battery provides a VIN = 5.6V to 8.4V. The loads require a

maximum of 600mA in active mode and 2mA in standby

mode. The output voltages are VOUT1 = 3.3V and VOUT2

= 2.5V. Since the load still needs power in standby, Burst

Mode operation is selected for good light load efficiency.

First, calculate the inductor values for about 240mA ripple

current at maximum VIN:

L1=

3.3V

2.25MHz •240mA

• 1–

3.3V

8.4V







 = 3.7µH

Choosingthecloseststandardizedinductorvalueof3.3μH

results in a maximum ripple current of:

∆IL1=

3.3V

2.25MHz •3.3µH

• 1– 3.3V

8.4V









=270mA

The same calculations for L2 result in a standard inductor

value of 3.3µH and a maximum current ripple of 236mA.

For cost reasons, a ceramic capacitor will be used. COUT

selection is then based on load step droop instead of ESR

requirements. For a 5% output droop:

COUT1≈5•

600mA

2.25MHz •(5%•3.3V)

=8.1 µF

COUT2≈5•

600mA

2.25MHz •(5%•2.5V)

=10.7 µF

For both outputs, a close standard value is 10µF. Since

the output impedance of a lithium-ion battery is very low,

each CIN is chosen to be 10µF also.

The output voltages can now be programmed by choosing

the values of R1 thru R4. To maintain high efficiency, the

current in these resistors should be kept small. Choosing

5µA with the 0.6V feedback voltage makes R2 and R4 ~

120k. Close standard 1% resistor values is 121k and then

R1 and R3 are 549k and 383k, respectively.

The PGOOD pins are common drain outputs, thus requir-

ing pull-up resistors. Two 100k resistors are used for

adequate speed.

Figure 1 shows the complete schematic for this design

example. The specific passive components chosen allow

for a 1mm height power supply that maintains a high ef-

ficiency across load.

Board Layout Considerations

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC3607. These items are also illustrated graphically

in the layout diagram of Figure 2. Check the following in

your layout:

1. Do the input capacitors CIN connect to PVIN1, PVIN2,

PGND1, and PGND2 as closely as possible? These ca-

pacitors provides the AC current to the internal power

MOSFETs and their drivers.

2. Are COUT and L closely connected? The (–) plate of

COUT returns current to GND and the (–) plate of CIN.

applicaTions inForMaTion

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