10

LTC1504A

output voltage is very close to ground. Under this condi-

tion, the LTC1504A must run at extremely narrow duty

cycles (< 5%) to keep the current under control. When the

on-time falls below the time required to sense the current

in Q1, the LTC1504A responds by reducing the oscillator

frequency, increasing the off-time to decrease the duty

cycle and allow it to maintain some control of the output

current. The oscillator frequency may drop by as much as

a factor of 10 under severe current overloads.

Under extreme short circuits (e.g., screwdriver to ground)

the on-time will reduce to the point where the LTC1504A

will lose control of the output current. At this point, output

current will rise until the inductor saturates, and the

current will be limited by the parasitic ESL of the inductor

usually nondestructive and dissipates a limited amount of

power since the output voltage is very low. A typical

LTC1504A circuit can withstand such a short for many

seconds without damage. The test circuit in Figure 1 will

typically withstand a direct output short for more than 30

seconds without damage to the LTC1504A. Eventually,

however, a continuous short may cause the die tempera-

ture to rise to destructive levels.

Note that the current limit is primarily designed to protect

the LTC1504A from damage and is not intended to be used

to generate an accurate constant-current output. As the

die temperature varies in a current limited condition, the

part to part due to manufacturing tolerance. The external

account for these variations without allowing the current

limit to engage at the maximum expected load current. A

current limit setting roughly double the expected load is

often a good compromise, eliminating unintended current

limit operation while preventing circuit destruction under

actual fault conditions. If desired, current limit can be

Shutdown

The LTC1504A includes a micropower shutdown mode

controlled by the logic level at SHDN. A logic High at SHDN

allows the part to operate normally. A logic Low at SHDN

stops all internal switching, pulls COMP, SS and SW to

GND and drops quiescent current below 1

µA typically.

Note that the internal N-channel power MOSFET from SW

to GND turns on when SHDN is asserted. This ensures that

the output voltage drops to zero when the LTC1504A is

shut down, but prevents other devices from powering the

output when the LTC1504A is disabled.

External Clock Synchronization

The LTC1504A SHDN pin can double as an external clock

input for applications that require a synchronized clock or

a faster switching speed. The SHDN pin terminates the

internal sawtooth wave and resets the oscillator immedi-

ately when it goes low, but waits 50

µs before shutting

down the rest of the internal circuitry. A clock signal

applied directly to the SHDN pin will force the LTC1504A

internal oscillator to lock to its frequency as long as the

external clock runs faster than the internal oscillator

frequency. Attempting to synchronize to a frequency

lower than the 250kHz maximum internal frequency may

result in inconsistent pulse widths and is not recom-

mended.

Because the sawtooth waveform rises at a fixed rate

internally, terminating it early by synchronizing to a fast

external clock will reduce the amplitude of the sawtooth

wave that the PWM comparator sees, effectively raising

the gain from COMP to SW. 500kHz is the maximum

recommended synchronization frequency; higher frequen-

cies will reduce the sawtooth amplitude to the point that

the LTC1504A may run erratically.

THERMAL CONSIDERATIONS

Each of the LTC1504A internal power switches has ap-

proximately 1.3

Ω of resistance at room temperature and

will happily carry more than the rated maximum current if

the current limit is set very high or is not connected. Since

the inductor current is always flowing through one or the

other of the internal switches, a typical application supply-

ing 500mA of load current will cause a continuous dissi-

pation of approximately 325mW. The SO-8 package has a

thermal resistance of approximately 90

°C/W, meaning

that the die will begin to rise toward 30

°C above ambient