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LT1185

APPLICATIO S I FOR ATIO

Large output capacitors (electrolytic or solid tantalum)

will not cause the LT1185 to oscillate, but they will cause

a damped “ringing” at light load currents where the ESR

of the capacitor is several orders of magnitude lower than

the load resistance. This ringing only occurs as a result of

transient load or line conditions and normally causes no

problems because of its low amplitude (

≤25mV).

Heat Sinking

The LT1185 will normally be used with a heat sink. The size

of the heat sink is determined by load current, input and

output voltage, ambient temperature, and the thermal

resistance of the regulator, junction-to-case (

θJC). The

LT1185 has two separate values for

θJC: one for the power

transistor section, and a second, lower value for the

control section. The reason for two values is that the

power transistor is capable of operating at higher continu-

ous temperature than the control circuitry. At low power

levels, the two areas are at nearly the same temperature,

and maximum temperature is limited by the control area.

At high power levels, the power transistor will be at a

significantly higher temperature than the control area

and its maximum operating temperature will be the

limiting factor.

To calculate heat sink requirements, you must solve a

thermal resistance formula twice, one for the power

transistor and one for the control area. The

lowest value

obtained for heat sink thermal resistance must be used. In

these equations, two values for maximum junction tem-

perature and junction-to-case thermal resistance are used,

as given in Electrical Specifications.

Example: A commercial version of the LT1185 in the

TO-220 package is to be used with a maximum ambient

temperature of 60

°C. Output voltage is 5V at 2A. Input

voltage can vary from 6V to 10V. Assume an interface

resistance of 1

°C/W.

First solve for control area, where the maximum junction

temperature is 125

°C for the TO-220 package, and

θJC = 1°C/W:

Next, solve for power transistor limitation, with

TJMAX = 150°C, θJC = 3°C/W:

The lowest number must be used, so heat sink resistance

must be less than 4.2

°C/W.

Some heat sink data sheets show graphs of heat sink

temperature rise vs power dissipation instead of listing a

value for thermal resistance. The formula for

θHS can be

rearranged to solve for maximum heat sink temperature

rise:

∆THS = TJMAX – TAMAX – P(θJC + θCHS)

Using numbers from the previous example:

∆THS = 125°C – 60 – 10.5(1 + 1) = 44°C control

section

∆THS = 150°C – 60 – 10.5(3 + 1) = 48°C power

transistor

The smallest rise must be used, so heat sink temperature

rise must be less than 44

°C at a power level of 10.5W.

For board level applications, where heat sink size may be

critical, one is often tempted to use a heat sink which

barely meets the requirements. This is permissible

correct assumptions were made concerning maximum

ambient temperature and power levels. One complicating

P = (10V – 5V) (2A) + 2A

(10V) = 10.5W

θHS = 125°C – 60°C

10.5W

– 1°C/W – 1°C/W = 4.2°C/W

θHS =

150 – 60

10.5

– 3 – 1 = 4.6°C/W

θHS =

(TJMAX – TAMAX)

–

θJC – θCHS.

θHS = Maximum heat sink thermal resistance

θJC = LT1185 junction-to-case thermal resistance

θCHS = Case-to-heat sink (interface) thermal

resistance, including any insulating washers

TJMAX = LT1185 maximum operating junction

temperature

TAMAX = Maximum ambient temperature in

customers application

P = Device dissipaton

= (VIN – VOUT) (IOUT) +

IOUT

(VIN)

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