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ISL97686
TABLE 2. PROTECTIONS TABLE
CASE
1
FAILURE MODE
CH1 Short Circuit
DETECTION MODE
Over-Temperature
FAILED CHANNEL ACTION
CH1 ON and burns power
GOOD CHANNELS ACTION
CH2 through CH4 Normal
V OUT
REGULATED BY
Highest VF of CH2
Protection limit (OTP) not
triggered and VCH1 < VSC
through CH4
2
CH1 Short Circuit
OTP not triggered but
VCH1 > VSC
CH1 disabled after 6 PWM cycles
time-out.
If 3 channels are already shut
down, all channels will be shut
Highest VF of CH2
through CH4
(Note: Time-out can be longer than 6
PWM cycles in direct PWM mode)
down. Otherwise CH2-4 will
remain as normal
3
4
CH1 Open Circuit with OTP not triggered and
infinite resistance VCH1 < VSC
CH1 Open Circuit with OTP triggered and
infinite resistance VCH1 < VSC
V OUT will ramp to OVP. CH1 will time-out CH2 through CH4 Normal
after 6 PWM cycles and switch off. V OUT
will drop to normal level.
All IC shut down
Highest VF of CH2
through CH4
V OUT disabled
during operation
5
CH1 LED Open Circuit OTP not triggered and
CH1 remains ON and has highest VF, CH2 through CH4 ON, Q2 through VF of CH1
but has paralleled
Zener
VCH1 < VSC
thus V OUT increases
Q4 burn power. CH2-4 will fault
out if they reach VSC as a result of
V OUT increase due to increase VF
in CH1
6
CH1 LED Open Circuit OTP not triggered but
but has paralleled VCHx > VSC
Zener
CH1 remains ON and has highest VF, V OUT increases then CH-X
thus V OUT increases. switches OFF. This is an unwanted
shut off and can be prevented by
VF of CH1
setting OVP at an appropriate
level.
7
Channel-to-Channel
Δ VF too high
OTP triggered but
VCHx < VSC
All channels switched off
V OUT disabled
8
9
Output LED string
voltage too high
V OUT /SW shorted to
GND
V OUT reaches OVP and not Driven with normal current. Any channel that is below the target current V OUT disabled
sufficient to regulate LED will time-out after 6 PWM cycles.
(Note: Time-out can be longer than 6 PWM cycles in case direct PWM
current
mode)
SW will not switch if started up in this condition. V OUT shorted to ground
during operation will also cause the converter to shut down
V L = L × Δ I L ? Δ t
Component Selections
According to the inductor Voltage-Second Balance principle, the
change of inductor current during the power MOSFET switching
on-time is equal to the change of inductor current during the
power MOSFET switching off-time under steady state operation.
The voltage across an inductor is shown in Equation 6:
(EQ. 6)
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
frequency of the loop causes almost all ripple current to flow in
the input capacitor, which must be rated accordingly.
A capacitor with low internal series resistance should be chosen
and Δ I L @ t ON = Δ I L @ t OFF , therefore:
( V I – 0 ) ? L × D × t Sw = ( V O – V D – V I ) ? L × ( 1 – D ) × t Sw
(EQ. 7)
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
V O ? V I = 1 ? ( 1 – D )
where D is the switching duty cycle defined by the turn-on time
over the switching period. V D is a Schottky diode forward voltage,
which can be neglected for approximation. t sw is the switching
period where t sw = 1/f sw , and the f sw is the switching frequency
of the boost converter.
Rearranging the terms without accounting for V D gives the boost
ratio and duty cycle respectively as Equations 8 and 9:
(EQ. 8)
other ceramic capacitors.
During the normal continuous conduction mode of the boost
converter, its 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
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
15
(EQ. 9)
FN7953.0
April 23, 2012
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