isl97636 Intersil Corporation, isl97636 Datasheet - Page 14

• Manufacturer Part Number: isl97636
• Description: 8-channel Led Driver
• Manufacturer: Intersil Corporation

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:
and ΔI
where D is the switching duty cycle defined by the turn-on
time over the switching period. V
voltage that can be neglected for approximation.
Rearranging the terms without accounting for V
boost ratio and duty cycle respectively as:
V
(
V
D
CASE
V
L
O
10
11
I
=
6
7
8
9
–
⁄
=
V
(
0 ) L ⁄
V
L
I
L
=
O
×
CH0 LED Open
Circuit but has
paralleled Zener
CH0 LED Open
Circuit but has
paralleled Zener
Channel-to-Channel
ΔVF too high
Channel-to-Channel
ΔVF too high
Output LED stack
voltage too high
VOUT/LX shorted to
GND
@ On = ΔI
1
ΔI
–
FAILURE MODE
×
⁄
V
L
(
D
1 D
I
⁄
) V
Δt
×
–
⁄
t
S
O
)
=
L
(
@ Off, therefore:
V
O
–
V
D
Upper OTP
triggered but
VIIN0 < VSC
Upper OTP not
triggered but
VIIN0 > VSC
Upper OTP not
triggered but VIINx
> VSC
Lower OTP
triggered but VIINx
< VSC
Upper OTP
triggered but VIINx
< VSC
VOUT > VOVP
LX current and
timing are
monitored.
OVP pin
monitored for
excursions below
20% of OVP
threshold
–
14
V
DETECTION
I
)
D
MODE
⁄
L
is Schottky diode forward
×
1 (
–
D )
TABLE 1. PROTECTIONS TABLE (Continued)
×
CH0 goes off until chip cooled and
then comes back on with current
reduced to 76%. Further OTP
triggers result in reduction to 53%,
then 30%.
CH0 OFF
CH0 remains ON and has highest
VF, thus VOUT increases.
Any channel at below 50% of the target current will fault out after 400µs.
Remaining channels driven with normal current.
All channels switched off until chip cooled and then comes back on with
current reduced to 76%. Further OTP triggers result in reduction to 53%,
then 30%.
Driven with normal current. Any channel that is below 50% of the target
current will time-out after 6ms.
Fault switch disabled and system shutdown until fault goes away, VOUT
is checked at start-up with a low current from LX to check for presence of
short before the fault switch is enabled.
t
S
D
FAILED CHANNEL ACTION
gives the
(EQ. 5)
(EQ. 6)
(EQ. 7)
(EQ. 8)
ISL97636
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, 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 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, with an AC ripple component proportional to the
rate of inductor charging only and 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.
Same as CH0
CH1 through CH7 Normal
VOUT increases then CH-X
switches OFF. This is an unwanted
shut off and the effect can be
minimized by setting OVP at an
appropriate level.
GOOD CHANNELS ACTION
VF of CH0
Highest VF of CH1
through CH7
VF of CH0
Highest VF of CH0
through CH7
Highest VF of CH0
through CH7
Highest VF of CH0
through CH7
REGULATED BY
VOUT
May 9, 2008
FN6570.0