LT1054CSW datasheet(9/16 Pages) LINER

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LT1054/LT1054L

APPLICATIONS INFORMATION

RX = VX/(4.4 IOUT)

where

VX ≈ VIN – [(LT1054 Voltage Loss)(1.3) + |VOUT|]

and IOUT = maximum required output current. The factor of

1.3 will allow some operating margin for the LT1054.

For example: assume a 12V to – 5V converter at 100mA

output current. First calculate the power dissipation with-

out an external resistor:

P = (12V –

|–5V|)(100mA) + (12V)(100mA)(0.2)

P = 700mW + 240mW = 940mW

θJA of 130°C/W for a commercial plastic device this

would cause a junction temperature rise of 122

°C so that

the device would exceed the maximum junction tempera-

ture at an ambient temperature of 25

°C. Now calculate the

power dissipation with an external resistor (RX). First find

how much voltage can be dropped across RX. The maxi-

mum voltage loss of the LT1054 in the standard regulator

configuration at 100mA output current is 1.6V, so

VX = 12V – [(1.6V)(1.3) + |–5V|] = 4.9V and

RX = 4.9V/(4.4)(100mA) = 11Ω

This resistor will reduce the power dissipated by the

LT1054 by (4.9V)(100mA) = 490mW. The total power

dissipated by the LT1054 would then be (940mW –

490mW) = 450mW. The junction temperature rise would

now be only 58

°C. Although commercial devices are

guaranteed to be functional up to a junction temperature

of 125

°C, the specifications are only guaranteed up to a

junction temperature of 100

°C, so ideally you should limit

the junction temperature to 100

°C. For the above example

this would mean limiting the ambient temperature to 42

°C.

Other steps can be taken to allow higher ambient tempera-

tures. The thermal resistance numbers for the LT1054

packages represent worst case numbers with no heat

sinking and still air. Small clip-on type heat sinks can be

used to lower the thermal resistance of the LT1054 pack-

age. In some systems there may be some available airflow

which will help to lower the thermal resistance. Wide PC

board traces from the LT1054 leads can also help to

remove heat from the device. This is especially true for

plastic packages.

where dV = peak-to-peak ripple and f = oscillator frequency.

For output capacitors with significant ESR a second term

must be added to account for the voltage step at the switch

transitions. This step is approximately equal to:

(2IOUT)(ESR of COUT)

Power Dissipation

The power dissipation of any LT1054 circuit must be

limited such that the junction temperature of the device

does not exceed the maximum junction temperature rat-

ings. The total power dissipation must be calculated from

two components, the power loss due to voltage drops in the

switches and the power loss due to drive current losses.

The total power dissipated by the LT1054 can be calculated

from:

≈ (VIN – |VOUT|)(IOUT) + (VIN)(IOUT)(0.2)

where both VIN and VOUT are referred to the ground pin (Pin

3) of the LT1054. For LT1054 regulator circuits, the power

dissipation will be equivalent to that of a linear regulator.

Due to the limited power handling capability of the LT1054

packages, the user will have to limit output current require-

ments or take steps to dissipate some power external to the

LT1054 for large input/output differentials. This can be

accomplished by placing a resistor in series with CIN as

shown in Figure 6. A portion of the input voltage will then

be dropped across this resistor without affecting the output

regulation. Because switch current is approximately 2.2

times the output current and the resistor will cause a

voltage drop when CIN is both charging and discharging,

the resistor should be chosen as:

CIN

COUT

VOUT

LT1054 • F06

VIN

LT1054

FB/SHDN

CAP+

GND

CAP –

V+

OSC

VREF

VOUT

Figure 6

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