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LT3507

3507f

The maximum allowed power dissipation by the LT3507

can be determined by:

PDISS(MAX) =

TJ(MAX) –TA

θJA

where TJMAX is the maximum die temperature of 125°C

(150°C for H grade).

However, take care in determining TA since the catch

diodes also dissipate power and must be located close

to the LT3507. Another potential heat source is the LDO

pass transistor. In a compact layout the pass transistor

will be located close to the LT3507. The inductors will

also dissipate some power due to their series resistance

and they must be close to the LT3507. All of these heat

sources will increase the effective ambient temperature

seen by the LT3507.

A thorough analysis of eight heat sources in a small PCB

area is beyond the scope of this data sheet, however a

number of thermal analysis programs are available to

calculate the temperature rise in each component (such

as PCAnalyze from K&K Associates or BETAsoft from

Mentor). The power dissipation of each component will be

needed to accurately calculate the thermal characteristics

of the system.

The contributors to power dissipation inside the LT3507 are

switch DC loss, switch AC loss, boost current, quiescent

current and LDO drive current. The total dissipation within

the LT3507 can be expressed as:

PDISS =

PSWDCi +PSWACi +PBSTi

()+PQ +PLDO

i=1

∑

The switch DC and AC losses in channel i are:

PSWDCi =

RSWi IOUTi

()

VOUTi

VINi

PSWACi = 17ns IOUTi

() VINi

() f()

APPLICATIONS INFORMATION

where RSWi is the equivalent switch resistance (0.18Ω for

channel 1 and 0.22Ω for channels 2 and 3) and f is the

operating frequency.

The boost loss in channel i is:

PBSTi =

VOUTi VBOOSTi

() IOUTi

+ 0.02A

VINi

The quiescent loss is:

PQ = VIN1(IQ(VIN1)) + VBIAS(IQ(BIAS))

If the BIAS pin does not have a voltage of at least 3V ap-

plied, then VIN1 must replace VBIAS in the equation. Also,

IQ(VIN1) can be reduced by 0.2mA (typ) if the LDO is shut

off (see the LDO section).

The LDO drive loss is:

PLDO =(VBIAS VLDO(OUT) 0.7V)

IOUT(LDO)

PASS

if VBIAS VLDO(OUT) + 1.5V

PLDO =(VIN1 VLDO(OUT) 0.7V)

IOUT(LDO)

PASS

if VBIAS <VLDO(OUT) + 1.5V

where βPASS is the current gain of the external pass

transistor.

Next, the power in the external components must be taken

into account. The diode power is given by:

PDIODE =

VF VIN –VOUT –VF

()IOUT

VIN

where VF is the forward drop of the diode at IOUT.

The inductor power is:

PIND = (IOUT)2 ESRIND

where ESRIND is the inductor equivalent series resistance.

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