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ISL97675
TABLE 3. PROTECTIONS TABLE (Continued)
GOOD CHANNELS
V OUT
REGULATED
CASE
FAILURE MODE
DETECTION MODE
FAILED CHANNEL ACTION
ACTION
BY
7
FB1 LED Open Circuit
but has paralleled
Zener
Upper OTP not triggered but FBx > 4V FB1 remains ON and has V OUT increases, then FB-X VF of FB1
highest VF, thus V OUT increases. switches OFF after 6
PWM cycles. This is an
unwanted shut off and
can be prevented by
setting OVP at an
appropriate level.
8
Channel-to-Channel
Δ VF too high
Lower OTP triggered but FBx < 4V
Any channel at below the target current will fault out after 6 Highest VF of FB1
through FB4
PWM cycles.
Remaining channels driven with normal current.
9
Channel-to-Channel
Δ VF too high
Upper OTP triggered but FBx < 4V
All channels go off until chip cooled and then comes back
on with current reduced to 76%. Subsequent OTP triggers
Highest VF of FB1
through FB4
will reduce I OUT further
10
Output LED stack
voltage too high
V OUT > VOVP
Any channel that is below the target current will time-out
after 6 PWM cycles, and V OUT will return to the normal
regulation voltage required for other channels.
Highest VF of FB1
through FB4
11
V OUT /LX shorted to LX current and timing are monitored. The chip is permanently shutdown 31ms after power-up if
GND at start-up or V OUT OVP pins monitored for excursions V OUT /Lx is shorted to GND.
shorted in operation below 20% of OVP threshold.
Components Selections
According to the inductor Voltage-Second Balance principle, the
change of inductor current during the switching regulator On-time is
equal to the change of inductor current during the switching
regulator Off-time. Since the voltage across an inductor is:
Input Capacitor
Switching regulators require input capacitors to deliver peak
charging current and to reduce the impedance of the input
supply. This reduces interaction between the regulator and input
supply, thereby improving system stability. The high switching
V L = L × Δ I L ? Δ t
(EQ. 13)
frequency of the loop causes almost all ripple current to flow in
the input capacitor, which must be rated accordingly.
( V I – 0 ) ? L × D × t S = ( V O – V D – V I ) ? L × ( 1 – D ) × t S
and Δ I L @ T ON = Δ I L @ T OFF , therefore:
(EQ. 14)
where D is the switching duty cycle defined by the turn-on time over
the switching period. V D is Schottky diode forward voltage which can
be neglected for approximation.
Rearranging the terms without accounting for V D gives the boost
ratio and duty cycle respectively as:
A capacitor with low internal series resistance should be chosen
to minimize heating effects and improve system efficiency, such
as X5R or X7R ceramic capacitors, which offer small size and a
lower value of temperature and voltage coefficient compared to
other ceramic capacitors.
In boost mode, input current flows continuously into the inductor;
AC ripple component is only proportional to the rate of the
inductor charging, thus, smaller value input capacitors may be
used. It is recommended that an input capacitor of at least 10μF
V O ? V I = 1 ? ( 1 – D )
(EQ. 15)
be used. Ensure the voltage rating of the input capacitor is
suitable to handle the full supply range.
D = ( V O – V I ) ? V O
17
(EQ. 16)
FN7630.1
September 14, 2011
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