LTC1504A
APPLICATIONS INFORMATION
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
and the R
ON
of Q2 inside the LTC1504A. This current is
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
R
ON
of the internal switches will change and the current
limit threshold will move around. R
ON
will also vary from
part to part due to manufacturing tolerance. The external
I
MAX
resistor should be chosen to allow enough room to
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
disabled by floating the I
MAX
pin; the internal current source
will pull I
MAX
to GND and the I
LIM
amplifier will be disabled.
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 1µA 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 50µs 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 1.3Ω 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
10
U
W
U
U
LINER [ LINEAR TECHNOLOGY ]
