LTC1430A Ver la hoja de datos (PDF) - Linear Technology
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LTC1430A Datasheet PDF : 24 Pages
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LTC1430A
APPLICATI S I FOR ATIO
LTC1430A will be in full operation. An internal switch
shorts the SS pin to GND during shutdown.
The LTC1430A detects the output current by watching the
voltage at IFB while Q1 is ON. The ILIM amplifier compares
this voltage to the voltage at IMAX (Figure 13). In the ON
state, Q1 has a known resistance; by calculating back-
wards, the voltage generated at IFB by the maximum
output current in Q1 can be determined. As IFB falls below
IMAX, ILIM will begin to sink current from the soft start pin,
causing the voltage at SS to fall. As SS falls, it will limit the
output duty cycle, limiting the current at the output.
Eventually the system will reach equilibrium, where the
pull-up current at the SS pin matches the pull-down
current in the ILIM amplifier; the LTC1430A will stay in this
state until the overcurrent condition disappears. At this
time IFB will rise, ILIM will stop sinking current and the
internal pull-up will recharge the soft start capacitor,
restoring normal operation. Note that the IFB pin requires
an external 1k series resistor to prevent voltage transients
at the drain of Q2 from damaging internal structures.
0.1µF
RIMAX
IMAX
12µA
ILIM
VCC
12µA
SS
CSS
PVCC
Q1
IFB 1k
Q2
LTC1430A
1430 F13
Figure 13. Current Limit Operation
The ILIM amplifier pulls current out of SS in proportion to
the difference between IFB and IMAX. Under mild overload
conditions, the SS pin will fall gradually, creating a time
delay before current limit takes effect. Very short, mild
overloads may not trip the current limit circuit at all.
Longer overload conditions will allow the SS pin to reach
a steady level, and the output will remain at a reduced
voltage until the overload is removed. Serious overloads
will generate a larger overdrive at ILIM, allowing it to pull SS
down more quickly and preventing damage to the output
components.
The ILIM amplifier output is disabled when Q1 is OFF to
prevent the low IFB voltage in this condition from activating
the current limit. It is re-enabled a fixed 170ns after Q1
turns on; this allows for the IFB node to slew back high and
the ILIM amplifier to settle to the correct value. As the
LTC1430A goes deeper into current limit, it will reach a
point where the Q1 on-time needs to be cut to below 170ns
to control the output current. This conflicts with the
minimum settling time needed for proper operation of the
ILIM amplifier. At this point, a secondary current limit
circuit begins to reduce the internal oscillator frequency,
lengthening the off-time of Q1 while the on-time remains
constant at 170ns. This further reduces the duty cycle,
allowing the LTC1430A to maintain control over the output
current.
Under extreme output overloads or short circuits, the ILIM
amplifier will pull the SS pin more than 2V below VCC in a
single switching cycle, cutting the duty cycle to zero. At
this point all switching stops, the output current decays
through Q2 and the LTC1430A runs a partial soft start
cycle and restarts. If the short is still present the cycle will
repeat. Peak currents can be quite high in this condition,
but the average current is controlled and a properly
designed circuit can withstand short circuits indefinitely
with only moderate heat rise in the output FETs. In addi-
tion, the soft start cycle repeat frequency can drop into the
low kHz range, causing vibrations in the inductor which
provide an audible alarm that something is wrong.
Oscillator Frequency
The LTC1430A includes an onboard current controlled
oscillator which will typically free-run at 200kHz. An
internal 20µA current is summed with any current in or out
of the FREQSET pin (Pin 11), setting the oscillator fre-
quency to approximately 10kHz/µA. FREQSET is internally
servoed to the LTC1430A reference voltage (1.265V).
With FREQSET floating, the oscillator is biased from the
15
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