LT1956IGN datasheet(20/28 Pages) LINER

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LT1956/LT1956-5

1956f

APPLICATIO S I FOR ATIO

Note: Some of the internal power dissipation in the IC, due

to BOOST pin voltage, can be transferred outside of the IC

to reduce junction temperature by increasing the voltage

drop in the path of the boost diode D2 (see Figure 9). This

reduction of junction temperature inside the IC will allow

higher ambient temperature operation for a given set of

conditions. BOOST pin circuitry dissipates power given by:

DISS

OUT

(BOOST Pin)

()

•/

•

Typically, VC2 (the boost voltage across the capacitor C2)

equals VOUT. This is because diodes D1 and D2 can be

considered almost equal, where:

VC2 = VOUT – VF(D2) – [–VF(D1)] = VOUT.

Hence, the equation for boost circuitry power dissipation

given in the previous Thermal Calculations section, is

stated as:

DISS BOOST

OUT

()

•/

•

()

Here it can be seen that boost power dissipation increases

as the square of VOUT. It is possible, however, to reduce

VC2 below VOUT to save power dissipation by increasing

the voltage drop in the path of D2. Care should be taken

that VC2 does not fall below the minimum 3.3V boost

voltage required for full saturation of the internal power

switch. For output voltages of 5V, VC2 is approximately 5V.

During switch turn on, VC2 will fall as the boost capacitor

C2 is discharged by the BOOST pin. In the previous BOOST

Pin section, the value of C2 was designed for a 0.7V droop

in VC2 (= VDROOP). Hence, an output voltage as low as 4V

would still allow the minimum 3.3V for the boost function

using the C2 capacitor calculated.

If a target output voltage of 12V is required, however, an

excess of 8V is placed across the boost capacitor which is

not required for the boost function but still dissipates

additional power.

What is required is a voltage drop in the path of D2 to

achieve minimal power dissipation while still maintaining

minimum boost voltage across C2.

A zener, D4, placed in series with D2 (see Figure 9), drops

voltage to C2.

Example:

The BOOST pin power dissipation for a 20V input to 12V

output conversion at 1A is given by:

BOOST =

()

12 1 36 12

•/

•

If a 7V zener is placed in series with D2, then power

dissipation becomes:

BOOST =

()

12 1 36 5

0 084

•/

•

For an FE package with thermal resistance of 45

°C/W,

ambient temperature savings would be:

T (ambient) savings = 0.116W • 45

°C/W = 5°C

For a GN package with thermal resistance of 85

°C/W,

ambient temperature savings would be:

T (ambient) savings = 0.116W • 85

°C/W = 10°C

The 7V zener should be sized for excess of 0.116W

operation. The tolerances of the zener should be consid-

ered to ensure minimum VBOOST exceeds 3.3V + VDROOP.

BOOST

VIN

VOUT

LT1956

SHDN

SYNC

BIAS

GND

1956 F09

VIN

Figure 9. BOOST Pin, Diode Selection

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