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ISL8201M
1. The maximum r DS(ON) at the highest junction
temperature
2. The minimum I SET from the “Electrical Specifications”
table on page 3.
Figure 19 shows the output response during a retry of an
output shorted to PGND. At time T 0 , the output has been
turned off due to sensing an overcurrent condition. There are
two internal soft-start delay cycles (T 1 and T 2 ) to allow the
I PEAK > I OUT ( MAX ) + -------------
3. Determine I PEAK for:
( Δ I L )
2
(EQ. 4)
MOSFETs to cool down in order to keep the average power
dissipation in retry at an acceptable level. At time T 2 , the
output starts a normal soft-start cycle, and the output tries to
where Δ I L is the output inductor ripple current. In a high input
voltage, high output voltage application, such as 20V input to
5V output, the inductor ripple becomes excessive due to the
fixed internal inductor value. In such applications, the output
current will be limited from the rating to approximately 70%
of the module’s rated current.
The relationships between the external R SET values and the
typical output current I OUT(MAX) OCP levels are as follows:
TABLE 2.
ramp. If the short is still applied and the current reaches the
I SET trip point any time during the soft-start ramp period, the
output will shut off and return to time T 0 for another delay
cycle. The retry period is thus two dummy soft-start cycles
plus one variable one (which depends on how long it takes to
trip the sensor each time). Figure 19 shows an example
where the output gets about half-way up before shutting
down; therefore, the retry (or hiccup) time will be around
17ms. The minimum should be nominally 13.6ms and the
maximum 20.4ms. If the short condition is finally removed,
the output should ramp up normally on the next T 2 cycle.
R SET
( Ω )
OPEN
50k
20k
10k
5k
3k
2k
OCP (A) @ V IN = 12V,
P VCC = 5V
13.3
12.6
11.4
10.2
7.6
6.3
4.9
OCP (A) @ V IN = 12V
P VCC = 12V
17.3
16.6
14.9
13.3
10.3
8.3
6.7
The range of allowable voltages detected (2 x I SET x R SET ) is
0mV to 475mV. If the voltage drop across R SET is set too
low, then this can cause almost continuous OCP tripping and
retry. It will also be very sensitive to system noise and inrush
current spikes, so it should be avoided. The maximum
usable setting is around 0.2V across R SET (0.4V across the
MOSFET); values above this might disable the protection.
Any voltage drop across R SET that is greater than 0.3V (0.6V
MOSFET trip point) will disable the OCP. Note that
conditions during power-up or during a retry may look
different than normal operation. During power-up in a 12V
system, the ISL8201M starts operation just above 4V; if the
supply ramp is slow, the soft-start ramp might be over well
before 12V is reached. Therefore, with low side gate drive
voltages, the r DS(ON) of the MOSFET will be higher during
power-up, effectively lowering the OCP trip. In addition, the
ripple current will likely be different at a lower input voltage.
Another factor is the digital nature of the soft-start ramp. On
each discrete voltage step, there is in effect, a small load
transient and a current spike to charge the output capacitors.
The height of the current spike is not controlled, however, it
is affected by the step size of the output and the value of the
output capacitors, as well as the internal error amp
compensation. Therefore, it is possible to trip the overcurrent
with inrush current, in addition to the normal load and ripple
considerations.
11
FIGURE 19. OVERCURRENT RETRY OPERATION
Starting up into a shorted load looks the same as a retry into
that same shorted load. In both cases, OCP is always
enabled during soft-start; once it trips, it will go into retry
(hiccup) mode. The retry cycle will always have two dummy
time-outs, plus whatever fraction of the real soft-start time
passes before the detection and shutoff. At that point, the
logic immediately starts a new two dummy cycle time-out.
Input Voltage Considerations
Figure 12 shows a standard configuration where P VCC is
either 5V (±10%) or 12V (±20%). In each case, the gate
drivers use the P VCC voltage for low side gate and high side
gate driver. In addition, P VCC is allowed to work anywhere
from 6.5V up to the 14.4V maximum. The P VCC range
between 5.5V and 6.5V is not allowed for long-term reliability
reasons, but transitions through it to voltages above 6.5V are
acceptable.
There is an internal 5V regulator for bias, which turns on
between 5.5V and 6.5V. Some of the delay after POR is there
to allow a typical power supply to ramp-up past 6.5V before
FN6657.2
October 21, 2010
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