ISL6534CRZ Intersil, ISL6534CRZ Datasheet - Page 24

• Manufacturer Part Number: ISL6534CRZ
• Description: IC CTRLR PWM DUAL LINEAR 32QFN
• Manufacturer: Intersil

Specifications of ISL6534CRZ

• Topology: Step-Down (Buck) Synchronous (2), Linear (LDO) (1)
• Function: Any Function
• Number Of Outputs: 3
• Frequency - Switching: 300kHz ~ 1MHz
• Voltage/current - Output 1: Controller
• Voltage/current - Output 2: Controller
• Voltage/current - Output 3: Controller
• W/led Driver: No
• W/supervisor: No
• W/sequencer: Yes
• Voltage - Supply: 3.3 V ~ 12 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: *
• Package / Case: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

detected. A comparator monitors the COMP pins, and if
either one exceeds the trip point (nominal 3.3V), and stays
above it for a filter time (1-2 clock pulses of the internal
oscillator; 3-6µs at the nominal 300kHz; 2-4µs at 500kHz),
then it will shut down both switchers, as well as the linear
regulator, and require a POR on either (or both) of the
VCC12 or VCC power pins. There is no “hiccup” or retry
mode, where it keeps trying. The protection was not
designed to work while powering up into a short circuit.
So that is the detection method; what are the implications of
it? On the plus side, it’s built in, and the user doesn’t have to
set anything to use it; no additional components are
required. On the negative side, it is not easy to predict its
performance, since many factors can affect how well it
works. It was designed to detect a “hard” short; like a
screwdriver shorting the output to GND. But defining how
close to “zero ohms” the short has to be in order to work
properly is not straightforward. If the resistance is too high to
trip the detector, the regulator will react simply as if the load
has increased, and will continue to try to regulate up until the
FETs overheat. If the COMP pin doesn’t immediately rise to
its trip point when the short is applied, chances are it won’t
trip later as the FETs heat up. So most of the potential
problems can occur if the initial trip is missed.
Following are a list of the many possible factors that affect
the performance:
1. If the power supply used for the VIN of one of the
2. If the power supply for VIN has a built-in current
3. If the circuit survives the initial short but doesn’t shut
4. The resistance of the short itself is probably the most
5. The higher the output voltage, the more current you will
regulators is shared with the VCC12 (or VCC) supply of
the IC, then shorting the output could potentially
momentarily drag down the supply low enough to trip the
VCC12 (or VCC) falling POR, which could result in
unpredictable behavior once the outputs shut off due to
the POR, and then try to start up into the short after the
supply recovers. This scenario can be avoided with a
“stiff” power supply, or a separate one.
shutdown or limit, then it might shut-down before the IC,
or the limiting might help the IC shutdown, either of which
is generally good. However, many supplies used in real
systems don’t have this built in, or would require a much
higher current short than this scenario would provide.
down, the removal of the short can cause an inductive
kick on the phase node, which can create an overvoltage
condition on the boot pin, which can in the worst case
damage the IC and/or the FETs.
critical factor affecting the overcurrent shutdown
performance. If the short is not low enough resistance,
then the part will NOT shutdown, and the FETs can
overheat. Note that the “short” to the output also includes
wiring, PCB traces, contact resistances, as well as all of
the return paths.
get out of a fixed-resistance short, and the more likely you
24
ISL6534
10. Other variables that may contribute to a lesser degree
11. Adding external circuitry to sense a fault may be possible,
So the recommendations are as follows:
6. In general, the faster the rise time of the output current
7. The load current at the time of the short can affect the
8. The compensation components are chosen to stabilize
9. The output capacitance and its ESR can affect how
1. If there is a specific fault condition that needs protection,
2. Compare the short circuit resistance to the nominal load
3. Check the rise time of the short circuit current, and what
4. From the waveform of the COMP pin, see if the values
will get a clean shutdown; see also #6. In addition, the
higher VOUT for a given VIN will give a higher UGATE
duty cycle, and the average COMP voltage is higher, so
it doesn’t have as far to go to trip.
during the short, the more current will be allowed on the
initial peak, and the better chance the COMP pin will have
a sharp rise as well. A low resistance short (#4) and a
higher output voltage (#5) both help. However, if the
current ramps too fast, then a false trip is also possible
(shutting down at a current level still within the expected
load range).
results; the response of a short can be different at no load
versus full load.
the regulation loop; however, if they unnecessarily load
the COMP output, that could affect the trip point
response.
quickly the current ramps up during a short.
include variations in the COMP comparator and filter, the
inductor L and DCR, the r
resistor dividers, the error amp reference voltage, the
oscillator frequency, switching noise, VCC voltage,
ambient temperature and airflow, and the layout of the
PCB.
but subject to the usual limitations of those circuits. For
example, sensing the output or FB voltage doesn’t
always directly correlate with output current.
try it out first under controlled conditions, either on an
EVAL board, the final circuit, or something close to it,
along with the power supply that will also be used.
Monitor VCC12 and VCC (to be sure they aren’t tripping
POR), the output and the COMP pin. A current probe
monitoring the output current is also very useful.
resistance; if they are too close, the circuit may not work
well. Calculate how long the FETs can sit at the higher
current. Is the short more likely from zero load or full
load?
happens if when the short is released.
can be optimized for the short condition. Within the
constraints of the stability criteria, smaller caps (in
general) may give a quicker response.
DS(ON)
of the FET, the FB
November 18, 2005
FN9134.2