LM2673 datasheet(12/26 Pages) NSC

12 / 26 page

Application Hints (Continued)

(which could saturate with excessively high currents) and the

catch diode be able to safely handle up to 5A which would be

conducted under load fault conditions.

If an application only requires a load current of 2A or so the

peak switch current can be set to a limit just over the maxi-

mum load current with the addition of a single programming

resistor. This allows the use of less powerful and more cost

effective inductors and diodes.

The peak switch current is equal to a factor of 37,125 divided

by R

ADJ. A resistance of 8.2K

Ω sets the current limit to

typically 4.5A. For predictable control of the current limit it is

recommended to keep the peak switch current greater than

1A. For lower current applications 500mA and 1A switching

regulators, the LM2674 and LM2672, are available.

When the power switch reaches the current limit threshold it

is immediately turned OFF and the internal switching fre-

quency is reduced. This extends the OFF time of the switch

to prevent a steady state high current condition. As the

switch current falls below the current limit threshold, the

switch will turn back ON. If a load fault continues, the switch

will again exceed the threshold and switch back OFF. This

will result in a low duty cycle pulsing of the power switch to

minimize the overall fault condition power dissipation.

Css SOFTSTART CAPACITOR

This optional capacitor controls the rate at which the LM2673

starts up at power on. The capacitor is charged linearly by an

internal current source. This voltage ramp gradually in-

creases the duty cycle of the power switch until it reaches

the normal operating duty cycle defined primarily by the ratio

of the output voltage to the input voltage. The softstart

turn-on time is programmable by the selection of Css.

The formula for selecting a softstart capacitor is:

Where:

SST = Softstart Current, 3.7µA typical

SS = Softstart time, from design requirements

SST = Softstart Threshold Voltage, 0.63V typical

OUT = Output Voltage, from design requirements

SCHOTTKY = Schottky Diode Voltage Drop, typically 0.5V

IN = Maximum Input Voltage, from design requirements

If this feature is not desired, leave the Softstart pin (pin 7)

open circuited

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

LM2673 (3.3V, 5V or 12V applications) or determine the

required feedback resistors for use with the adjustable

LM2673−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.

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