LM3554 datasheet(34/47 Pages) TI1

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LED

LOAD =

IND

LED

OUT +

x ILED

IND

)

- V

IND

(

+ VOUT

PFET

LOAD

DISS

) x

- V

OUT

NFET

LOAD

VOUT

)

- V

LED

OUT

IND

LED

DISS

Maximum Output Current vs Efficiency

0.6

0.7

0.8

0.9

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.1

2.2

2.3

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

VIN = 3.6V, VOUT = 4V

VIN = 3V, VOUT = 4V

VIN = 3.6V, VOUT = 5V

VIN = 2.5V, VOUT = 4V

VIN = 3V, VOUT = 5V

VIN = 2.5V, VOUT = 5V

Efficiency (P

OUT /PIN )

(IPEAK = 2.5A)

LM3554

SNVS549C – JUNE 2009 – REVISED FEBRUARY 2016

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Figure 44. LM3554 Maximum Output Current

8.2.2.7 Maximum Output Power

Output power is limited by three things: the peak current limit, the ambient temperature, and the maximum power

dissipation in the package. If the LM3554’s die temperature is below the absolute maximum rating of 125°C, the

maximum output power can be over 6 W. However, any appreciable output current causes the internal power

dissipation to increase and therefore increase the die temperature. This can be additionally compounded if the

LED current sources are operating while the device is in voltage-output mode because the difference between

VOUT and VLED is dropped across the current sources. Any circuit configuration must ensure that the die

temperature remains below 125°C taking into account the ambient temperature derating.

8.2.2.7.1

Voltage-Output Mode

In voltage-output mode the total power dissipated in the LM3554 can be approximated as:

where

•

PN is the power lost in the NFET

•

PP is the PFET power loss

•

PLED1, PLED2, and PIND are the losses across the current sink

(10)

An approximate calculation of these losses gives:

(11)

Equation 11 consider the average current through the NFET and PFET. The actual power losses are higher due

to the RMS currents and the quiescent power into IN. These, however, can give a decent approximation.

8.2.2.7.2

LED Boost Mode

In LED mode with VOUT > VIN the device boost converter switches and make VOUT = VLED + 0.3 V. In this situation

the total power dissipated in the LM3554 is approximated as:

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