LTC3621 datasheet(14/20 Pages) AD

14 / 20 page

LTC3644/LTC3644-2

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

during dead-time and inductor core losses, generally

account for less than 2% total additional loss.

Thermal Conditions

In a majority of applications, the LTC3644 does not

dissipate much heat due to its high efficiency. However,

in applications where the LTC3644 is running at high

ambient temperature, high VIN, high switching frequency,

andmaximumoutputcurrentload,theheatdissipatedmay

exceed the maximum junction temperature of the part. If

the junction temperature reaches approximately 160°C,

all power switches will be turned off until the temperature

drops about 15°C cooler.

To avoid the LTC3644 from exceeding the maximum

junction temperature, the user 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 temperature rise is

given by:

TRISE = PD • θJA

As an example, consider the case when the LTC3644 is

used in applications where VIN = 12V, IOUT1 = IOUT2 = IOUT3

= IOUT4 = 0.8A, f = 1MHz, VOUT = 1.8V. The equivalent

power MOSFET resistance RSW is:

RSW =RDS(ON)TOP •

VOUT

VIN

+RDS(ON)BOT • 1−

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

= 300mΩ•

1.8V

12V

+80mΩ• 1−

1.8V

12V

⎛

⎝⎜

⎞

⎠⎟

= 113mΩ

The active current through VIN at 1MHz without load is

about 5mA, which includes switching and internal biasing

currentloss,andtransitionloss.Therefore,thetotalpower

dissipated by the part is:

PD = 4 • IOUT2 • RSW + VIN • IIN(Q)

= 4 • 0.8A2 • 113mΩ + 12V • 5mA

= 349mW

For the BGA package, the θJA is 25°C/W as measured

on the LTC3644 demo board. Therefore, the junction

temperature of the regulator operating at 25°C ambient

temperature is approximately:

TJ = 349mW • 25°C/W + 25°C = 33.7°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. Redoing the calculation

assuming that RSW increased 5% at 33.7°C yields a new

junction temperature of 34.1°C. If the application calls

for a higher ambient temperature and/or higher switching

frequency, care should be taken to reduce the temperature

rise of the part by using a heat sink or airflow.

Board Layout Considerations

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC3644 (refer to Figure 4). Check the following in

the layout:

1. Do the capacitors CIN connect to the VIN and GND as

close as possible? These capacitors provide the AC

currenttotheinternalpowerMOSFETsandtheirdrivers.

Does CVCC connect to INTVCC as close as possible?

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

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

3. The resistive divider, R1 and R2, must be connected

between the (+) plate of COUT and a ground line

terminated near GND. The feedback signal VFB should

be routed away from noisy components and traces,

such as the SW line, and its trace length should be

minimized. Keep R1 and R2 close to the IC.

4. KeepsensitivecomponentsawayfromtheSWpin.The

input capacitor, CIN, feedback resistors, and INTVCC

bypass capacitors should be routed away from the

SW trace and the inductor.

5. Agroundplaneispreferred.Useseveralviasconnected

to ground on the component side.

6. Flood all unused areas on all layers with copper, which

reduces the temperature rise of power components.

These copper areas should be connected to GND.

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