LM2670 datasheet(13/26 Pages) NSC

13 / 26 page

Application Hints (Continued)

pin 5. This RC network creates a short 100nS pulse on each

positive edge of the clock to reset the internal ramp oscilla-

tor. The reset time of the oscillator is approximately 300nS.

ADDITIONAL APPLICATION INFORMATION

When the output voltage is greater than approximately 6V,

and the duty cycle at minimum input voltage is greater than

approximately 50%, the designer should exercise caution in

selection of the output filter components. When an applica-

tion designed to these specific operating conditions is sub-

jected to a current limit fault condition, it may be possible to

observe a large hysteresis in the current limit. This can affect

the output voltage of the device until the load current is

reduced sufficiently to allow the current limit protection circuit

to reset itself.

Under current limiting conditions, the LM267x is designed to

respond in the following manner:

At the moment when the inductor current reaches the

current limit threshold, the ON-pulse is immediately ter-

minated. This happens for any application condition.

However, the current limit block is also designed to

momentarily reduce the duty cycle to below 50% to

avoid subharmonic oscillations, which could cause the

inductor to saturate.

Thereafter, once the inductor current falls below the

current limit threshold, there is a small relaxation time

during which the duty cycle progressively rises back

above 50% to the value required to achieve regulation.

If the output capacitance is sufficiently ‘large’, it may be

possible that as the output tries to recover, the output ca-

pacitor charging current is large enough to repeatedly re-

trigger the current limit circuit before the output has fully

settled. This condition is exacerbated with higher output

voltage settings because the energy requirement of the out-

put capacitor varies as the square of the output voltage

(1⁄2CV

2), thus requiring an increased charging current.

A simple test to determine if this condition might exist for a

suspect application is to apply a short circuit across the

output of the converter, and then remove the shorted output

condition. In an application with properly selected external

components, the output will recover smoothly.

Practical values of external components that have been

experimentally found to work well under these specific oper-

ating conditions are C

OUT = 47µF, L = 22µH. It should be

noted that even with these components, for a device’s cur-

rent limit of I

CLIM, the maximum load current under which the

possibility of the large current limit hysteresis can be mini-

mized is I

CLIM/2. For example, if the input is 24V and the set

output voltage is 18V, then for a desired maximum current of

1.5A, the current limit of the chosen switcher must be con-

firmed to be at least 3A.

SIMPLE DESIGN PROCEDURE

Using the nomographs and tables in this data sheet (or use

the available design software at http://www.national.com) a

complete step-down regulator can be designed in a few

simple steps.

Step 1: Define the power supply operating conditions:

Required output voltage

Maximum DC input voltage

Maximum output load current

Step 2: Set the output voltage by selecting a fixed output

LM2670 (3.3V, 5V or 12V applications) or determine the

required feedback resistors for use with the adjustable

LM2670−ADJ

Step 3: Determine the inductor required by using one of the

four nomographs, Figure 3 through Figure 6. Table 1 pro-

vides a specific manufacturer and part number for the induc-

tor.

Step 4: Using Table 3 (fixed output voltage) or Table 6

(adjustable output voltage), determine the output capaci-

tance required for stable operation. Table 2 provides the

specific capacitor type from the manufacturer of choice.

Step 5: Determine an input capacitor from Table 4 for fixed

output voltage applications. Use Table 2 to find the specific

capacitor type. For adjustable output circuits select a capaci-

tor from Table 2 with a sufficient working voltage (WV) rating

greater than Vin max, and an rms current rating greater than

one-half the maximum load current (2 or more capacitors in

parallel may be required).

Step 6: Select a diode from Table 5. The current rating of the

diode must be greater than I load max and the Reverse

Voltage rating must be greater than Vin max.

Step 7: Include a 0.01µF/50V capacitor for Cboost in the

design.

FIXED OUTPUT VOLTAGE DESIGN EXAMPLE

A system logic power supply bus of 3.3V is to be generated

from a wall adapter which provides an unregulated DC volt-

age of 13V to 16V. The maximum load current is 2.5A.

Through-hole components are preferred.

Step 1: Operating conditions are:

Vout = 3.3V

Vin max = 16V

Iload max = 2.5A

Step 2: Select an LM2670T-3.3. The output voltage will have

a tolerance of

±2% at room temperature and ±3% over the full operating

temperature range.

Step 3: Use the nomograph for the 3.3V device ,Figure 3.

The intersection of the 16V horizontal line (V

in max) and the

2.5A vertical line (I

load max) indicates that L33, a 22µH

inductor, is required.

From Table 1, L33 in a through-hole component is available

from Renco with part number RL-1283-22-43 or part number

PE-53933 from Pulse Engineering.

Step 4: Use Table 3 to determine an output capacitor. With a

3.3V output and a 22µH inductor there are four through-hole

output capacitor solutions with the number of same type

capacitors to be paralleled and an identifying capacitor code

given. Table 2 provides the actual capacitor characteristics.

Any of the following choices will work in the circuit:

1 x 220µF/10V Sanyo OS-CON (code C5)

1 x 1000µF/35V Sanyo MV-GX (code C10)

1 x 2200µF/10V Nichicon PL (code C5)

1 x 1000µF/35V Panasonic HFQ (code C7)

Step 5: Use Table 4 to select an input capacitor. With 3.3V

output and 22µH there are three through-hole solutions.

These capacitors provide a sufficient voltage rating and an

rms current rating greater than 1.25A (1/2 I

load max). Again

using Table 2 for specific component characteristics the

following choices are suitable:

1 x 1000µF/63V Sanyo MV-GX (code C14)

1 x 820µF/63V Nichicon PL (code C24)

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