LM3554 datasheet(19/43 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

www.ti.com

SNVS549A – JUNE 2009 – REVISED APRIL 2011

Figure 3. LM3554 Maximum Output Current

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 6W. However, any appreciable output current will cause 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 since 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.

Maximum Output Power (Voltage Output Mode)

In Voltage Output mode the total power dissipated in the LM3554 can be approximated as:

(3)

PN is the power lost in the NFET, PP is the PFET power loss, PLED1, PLED2, and PIND are the losses across the

current sinks. An approximate calculation of these losses gives:

(4)

The above formulas consider the average current through the NFET and PFET. The actual power losses will be

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

approximation.

Maximum Output Power (Led Boost Mode)

In LED mode with VOUT > VIN the LM3554’s boost converter will switch and make VOUT = VLED + 0.3V. In this

situation the total power dissipated in the LM3554 is approximated as:

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