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LT1941

Triple Monolithic Switching Regulator

Linear Technology 

LT1941

APPLICATIONS INFORMATION

your load is lower than the maximum load current, then

you can relax the value of the inductor and operate with

higher ripple current. This allows you to use a physically

smaller inductor or one with a lower DCR resulting in

higher efficiency. Be aware that if the inductance differs

from the simple rule above, then the maximum load current

will depend on input voltage. In addition, low inductance

may result in discontinuous mode operation, which further

reduces maximum load current. For details of maximum

output current and discontinuous mode operation, see

Linear Technology’s Application Note AN44. Finally, for

duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum

inductance is required to avoid subharmonic oscillations.

See AN19.

The current in the inductor is a triangle wave with an average

value equal to the load current. The peak switch current

is equal to the output current plus half the peak-to-peak

inductor ripple current. The LT1941 limits its switch cur-

rent in order to protect itself and the system from overload

faults. Therefore, the maximum output current that the

LT1941 will deliver depends on the switch current limit,

the inductor value and the input and output voltages.

When the switch is off, the potential across the inductor

is the output voltage plus the catch diode drop. This gives

the peak-to-peak ripple current in the inductor:

ΔIL = (1 – DC)(VOUT + VF)/(L • f)

where f is the switching frequency of the LT1941 and L

is the value of the inductor. The peak inductor and switch

current is:

ISWPK = ILPK = IOUT + ΔIL/2

To maintain output regulation, this peak current must be

less than the LT1941’s switch current limit ILIM. For SW1,

ILIM is at least 3A at low duty cycles and decreases linearly

to 2.4A at DC = 0.8. For SW2, ILIM is at least 2A for at low

duty cycles and decreases linearly to 1.6A at DC = 0.8.

The maximum output current is a function of the chosen

inductor value:

IOUT(MAX) = ILIM – ΔIL/2

= 3 • (1 – 0.25 • DC) – ΔIL/2 for SW1

= 2 • (1 – 0.25 • DC) – ΔIL/2 for SW2

Choosing an inductor value so that the ripple current is

small will allow a maximum output current near the switch

current limit.

One approach to choosing the inductor is to start with the

simple rule given above, look at the available inductors

and choose one to meet cost or space goals. Then use

these equations to check that the LT1941 will be able to

deliver the required output current. Note again that these

equations assume that the inductor current is continu-

ous. Discontinuous operation occurs when IOUT is less

than ΔIL/2.

Output Capacitor Selection

For 5V and 3.3V outputs, a 10μF, 6.3V ceramic capacitor

(X5R or X7R) at the output results in very low output volt-

age ripple and good transient response. For lower voltages,

10μF is adequate for ripple requirements but increasing

COUT will improve transient performance. Other types and

values will also work; the following discusses tradeoffs in

output ripple and transient performance.

The output capacitor filters the inductor current to generate

an output with low voltage ripple. It also stores energy in

order to satisfy transient loads and stabilize the LT1941’s

control loop. Because the LT1941 operates at a high

frequency, minimal output capacitance is necessary. In

addition, the control loop operates well with or without

the presence of output capacitor series resistance (ESR).

Ceramic capacitors, which achieve very low output ripple

and small circuit size, are therefore an option.

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