LT1940 Ver la hoja de datos (PDF) - Linear Technology

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LT1940

Dual Monolithic 1.4A, 1.1MHz Step-Down Switching Regulator

Linear Technology 

LT1940/LT1940L

APPLICATIO S I FOR ATIO

is likely to see high surge currents when the input source

is applied, tantalum capacitors should be surge rated. The

manufacturer may also recommend operation below the

rated voltage of the capacitor. Be sure to place the 1µF

ceramic as close as possible to the VIN and GND pins on

the IC for optimal noise immunity.

A final caution is in order regarding the use of ceramic

capacitors at the input. A ceramic input capacitor can

combine with stray inductance to form a resonant tank

circuit. If power is applied quickly (for example by plug-

ging the circuit into a live power source) this tank can ring,

doubling the input voltage and damaging the LT1940. The

solution is to either clamp the input voltage or dampen the

tank circuit by adding a lossy capacitor in parallel with the

ceramic capacitor. For details, see AN88.

Output Capacitor Selection

For 5V and 3.3V outputs with greater than 1A output, a

10µF 6.3V ceramic capacitor (X5R or X7R) at the output

results in very low output voltage ripple and good transient

response. For lower voltages, 10µF is adequate but in-

creasing COUT to 15µF or 22µF will improve transient

performance. Other types and values can be used; the

following discusses tradeoffs in output ripple and tran-

sient performance.

The output capacitor filters the inductor current to gener-

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

in order satisfy transient loads and to stabilize the LT1940’s

control loop. Because the LT1940 operates at a high

frequency, you don’t need much output capacitance. Also,

the current mode control loop doesn’t require the pres-

ence of output capacitor series resistance (ESR). For these

reasons, you are free to use ceramic capacitors to achieve

very low output ripple and small circuit size.

Estimate output ripple with the following equations:

VRIPPLE = ∆IL/(8f COUT) for ceramic capacitors, and

VRIPPLE = ∆IL ESR for electrolytic capacitors (tantalum

and aluminum);

where ∆IL is the peak-to-peak ripple current in the induc-

tor. The RMS content of this ripple is very low, and the

RMS current rating of the output capacitor is usually not

of concern.

Another constraint on the output capacitor is that it must

have greater energy storage than the inductor; if the stored

energy in the inductor is transferred to the output, you

would like the resulting voltage step to be small compared

to the regulation voltage. For a 5% overshoot, this require-

ment becomes COUT > 10L(ILIM/VOUT)^2.

Finally, there must be enough capacitance for good tran-

sient performance. The last equation gives a good starting

point. Alternatively, you can start with one of the designs

in this data sheet and experiment to get the desired

performance. This topic is covered more thoroughly in the

section on loop compensation.

The high performance (low ESR), small size and robust-

ness of ceramic capacitors make them the preferred type

for LT1940 applications. However, all ceramic capacitors

are not the same. As mentioned above, many of the higher

value capacitors use poor dielectrics with high tempera-

ture and voltage coefficients. In particular, Y5V and Z5U

types lose a large fraction of their capacitance with applied

voltage and temperature extremes. Because the loop

stability and transient response depend on the value of

COUT, you may not be able to tolerate this loss. Use X7R

and X5R types.

You can also use electrolytic capacitors. The ESRs of most

aluminum electrolytics are too large to deliver low output

ripple. Tantalum and newer, lower ESR organic electro-

lytic capacitors intended for power supply use are suit-

able, and the manufacturers will specify the ESR. The

choice of capacitor value will be based on the ESR required

for low ripple. Because the volume of the capacitor deter-

mines its ESR, both the size and the value will be larger

than a ceramic capacitor that would give you similar ripple

performance. One benefit is that the larger capacitance

may give better transient response for large changes in

load current. Table 2 lists several capacitor vendors.

1940fa

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