LTC3561 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LTC3561

1A, 4MHz, Synchronous Step-Down DC/DC Converter

Linear Technology 

LTC3561

APPLICATIO S I FOR ATIO

looking into the SW pin is a function of both top and

bottom MOSFET RDS(ON) and the duty cycle (DC) as

follows:

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

The RDS(ON) for both the top and bottom MOSFETs can

be obtained from the Typical Performance Characteris-

tics curves. Thus, to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

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

frequency. Other losses including diode conduction

losses during dead-time and inductor core losses gen-

erally account for less than 2% total additional loss.

Thermal Considerations

In a majority of applications, the LTC3561 does not dissi-

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

applications where the LTC3561 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 be turned off and the SW node will become

high impedance.

To avoid the LTC3561 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 maxi-

mum junction temperature of the part. The temperature

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 LTC3561 is in

dropout at an input voltage of 3.3V with a load current of

1A. From the Typical Performance Characteristics graph

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

P-channel switch is 0.11Ω. Therefore, power dissipated

by the part is:

PD = I2 • RDS(ON) = 110mW

The DD8 package junction-to-ambient thermal resistance,

θJA, will be in the range of about 43°C/W. Therefore, the

junction temperature of the regulator operating in a 70°C

ambient temperature is approximately:

TJ = 0.11 • 43 + 70 = 74.7°C

Remembering that the above junction temperature is

obtained from an RDS(ON) at 25°C, we might recalculate

the junction temperature based on a higher RDS(ON) since

it increases with temperature. However, we can safely

assume that the actual junction temperature will not

exceed the absolute maximum junction temperature of

125°C.

Design Example

As a design example, consider using the LTC3561 in a

portable application with a Li-Ion battery (refer to Figure 4

for reference designation). The battery provides a VIN =

2.5V to 4.2V. The load requires a maximum of 1A in active

mode and 10mA in standby mode. The output voltage is

VOUT = 2.5V.

First, calculate the timing resistor:

RT = 9.78 • ( ) 1011 1MHz −1.08 = 323.8k

Use a standard value of 324k. Next, calculate the inductor

value for about 40% ripple current at maximum VIN:

L

=

2.5V

1MHz • 400mA

•

⎛

⎝⎜

1−

2.5V

4.2V

⎞

⎠⎟

=

2.5µH

Choosing the closest inductor from a vendor of 2.2µH,

results in a maximum ripple current of:

∆IL

=

2.5V

1MHz • 2.2µ

• ⎛⎝⎜1−

2.5V

4.2V

⎞⎠⎟

=

460mA

3561f

11

Share Link: