LTC3548-2(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC3548-2
(Rev.:RevA)

Dual Synchronous, Fixed/Adjustable Output, 2.25MHz Step-Down DC/DC Regulator

Linear Technology 

LTC3548-2

APPLICATIO S I FOR ATIO

4) Other “hidden” losses such as copper trace and internal

battery resistances can account for additional efficiency

degradations in portable systems. It is very important to

include these “system” level losses in the design of a

system. The internal battery and fuse resistance losses

can be minimized by making sure that CIN has adequate

charge storage and very low ESR at the switching fre-

quency. Other losses including diode conduction losses

during dead-time and inductor core losses generally ac-

count for less than 2% total additional loss.

Thermal Considerations

In a majority of applications, the LTC3548-2 does not

dissipate much heat due to its high efficiency. However, in

applications where the LTC3548-2 is running at high

ambient temperature with low supply voltage and high

duty cycles, such as in dropout, the heat dissipated may

exceed the maximum junction temperature of the part. If

the junction temperature reaches approximately 150°C,

both power switches will turn off and the SW node will

become high impedance.

To prevent the LTC3548-2 from exceeding the maximum

junction temperature, the user will need to do some

thermal analysis. The goal of the thermal analysis is to

determine whether the power dissipated exceeds the

maximum junction temperature of the part. The tempera-

ture rise is given by:

TRISE = PD • θJA

where PD is the power dissipated by the regulator and θJA

is the thermal resistance from the junction of the die to the

ambient temperature.

The junction temperature, TJ, is given by:

TJ = TRISE + TAMBIENT

As an example, consider the case when the LTC3548-2 is

at an input voltage of 2.7V with a load current of 400mA

and 800mA and an ambient temperature of 70°C. From

the Typical Performance Characteristics graph of Switch

Resistance, the RDS(ON) resistance of the main switch is

0.425Ω. Therefore, power dissipated by each channel is:

PD = I2 • RDS(ON) = 272mW and 68mW

12

The DFN package junction-to-ambient thermal resistance,

θJA, is 40°C/W. Therefore, the junction temperature of the

regulator operating in a 70°C ambient temperature is

approximately:

TJ = (0.272 + 0.068) • 40 + 70 = 83.6°C

which is below the absolute maximum junction tempera-

ture of 125°C.

Design Example

As a design example, consider using the LTC3548-2 in an

portable application with a Li-Ion battery. The battery

provides a VIN = 2.8V to 4.2V. The load requires a maxi-

mum of 800mA in active mode and 2mA in standby mode.

The output voltage is VOUT1 = 1.8V. Since the load still

needs power in standby, Burst Mode operation is selected

for good low load efficiency.

First, calculate the inductor value for about 30% ripple

current at maximum VIN:

L

=

1.8V

2.25MHz • 240mA

•

⎛

⎝⎜

1–

1.8V

4.2V

⎞

⎠⎟

=

1.9µH

Choosing a vendor’s closest inductor value of 2.2µH,

results in a maximum ripple current of:

∆IL

=

1.8V

2.25MHz •

2.2µ

•

⎛

⎝⎜

1−

1.8V

4.2V

⎞

⎠⎟

=

208mA

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:

COUT

≈

2.5

800mA

2.25MHz •(5%

•

1.8V)

=

9.9µF

A good standard value is 10µF. Since the output imped-

ance of a Li-Ion battery is very low, CIN is typically 10µF.

Following the same procedure for VOUT2 = 2.5V, the

inductor value is derived as 4.7µH and the output capacitor

value is 4.7µF.

The output voltage, VOUT2, can now be programmed by

choosing the values of R1 and R2. To maintain high

efficiency, the current in these resistors should be kept

small. Choosing 2µA with the 0.6V feedback voltage

35482fa

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