LTC3636 datasheet(20/28 Pages) LINER

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LTC3636/LTC3636-1

3636fb

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APPLICATIONS INFORMATION

which is below the maximum junction temperature of

125°C. With higher ambient temperatures, a heat sink or

cooling fan should be considered to drop the junction-to-

ambient thermal resistance.

Remembering that the above junction temperature is

obtained from an RDS(ON) at 50°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 15% at 87°C yields a new

junction temperature of 92°C. If the application calls for a

higher ambient temperature and/or higher load currents,

care should be taken to reduce the temperature rise of the

part by using a heat sink or air flow.

Figure 7 is a temperature derating curve based on the

DC2335 demo board (QFN package). It can be used to

estimate the maximum allowable ambient temperature

for given DC load currents in order to avoid exceeding

the maximum operating junction temperature of 125°C.

Figure 7. Temperature Derating Curve for DC2335 Demo Circuit

2) The output capacitor, COUT, and inductor L should be

closely connected to minimize loss. The (–) plate of

COUT should be closely connected to both GND and

the (–) plate of CIN.

3) The resistive divider, (e.g. R1 to R4 in Figure 8) 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) Keep sensitive components away from the SW pin. The

RTresistor,thecompensationcomponents,thefeedback

resistors, and the INTVCC bypass capacitor should all

be routed away from the SW trace and the inductor L.

5) A ground plane is preferred.

6) Flood all unused areas on all layers with copper in order

to reduce the temperature rise of power components.

Thesecopperareasshouldbeconnectedtotheexposed

backside of the package (GND).

Refer to Figure 9 for board layout examples.

Design Example

As a design example, consider using the LTC3636/

LTC3636-1 in an application with the following specifi-

cations: VIN(MAX) = 13.2V, VOUT1 = 1.8V, VOUT2 = 3.3V,

IOUT(MAX) = 6A, IOUT(MIN) = 10mA, f = 2MHz, VDROOP ~

(5% • VOUT). The following discussion will use equations

from the previous sections.

Because efficiency is important at both high and low load

current, Burst Mode operation will be utilized.

First, the correct RT resistor value for 2MHz switching fre-

quency must be chosen. Based on the equation discussed

earlier, RT should be 160k; the closest standard value is

162k. RT can be tied to INTVCC if switching frequency

accuracy is not critical.

Next, determine the channel 1 inductor value for about

40% ripple current at maximum VIN:

L1=

1.8V

2MHz • 2.4A

⎛

⎝

⎜

⎞

⎠

⎟ 1−

1.8V

13.2V

⎛

⎝

⎜

⎞

⎠

⎟ = 0.32µH

VIN = 12V

VOUT1 = 1.8V

VOUT2 = 3.3V

fSW = 1MHz

CH1 LOAD = 0A

CH1 LOAD = 2A

CH1 LOAD = 4A

CH1 LOAD = 6A

MAXIMUM ALLOWABLE AMBIENT TEMPERATURE (°C)

100

125

1.0

2.0

3.0

4.0

5.0

6.0

7.0

3636 F07

Board Layout Considerations

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC3636/LTC3636-1. Check the following in your layout:

1) Do the input capacitors connect to the VIN and GND

pins as close as possible? These capacitors provide

the AC current to the internal power MOSFETs and their

drivers.

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