ISL97672IRZ Intersil, ISL97672IRZ Datasheet - Page 14

• Manufacturer Part Number: ISL97672IRZ
• Description: IC LED DVR PWM CTRL 6CH 20QFN
• Manufacturer: Intersil

Specifications of ISL97672IRZ

• Constant Current: *
• Constant Voltage: *
• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 6
• Internal Driver: Yes
• Type - Primary: Automotive, Backlight
• Frequency: 200kHz ~ 1.5MHz
• Voltage - Supply: 4.5 V ~ 26.5 V
• Voltage - Output: *
• Mounting Type: Surface Mount
• Package / Case: 20-VFQFN Exposed Pad
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 40mA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
forward voltage that can be neglected for approximation.
Rearranging the terms without accounting for V
the boost ratio and duty cycle respectively as Equations 8
and 9:
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.
Inductor
The selection of the inductor should be based on its
maximum current (I
dissipation (DCR), EMI susceptibility (shielded vs
unshielded), and size. Inductor type and value influence
many key parameters, including ripple current, current
limit, efficiency, transient performance and stability.
Its maximum current capability must be adequate to
handle the peak current at the worst case condition. If an
inductor core is chosen with too low a current rating,
saturation in the core will cause the effective inductor
value to fall, leading to an increase in peak to average
current level, poor efficiency and overheating in the core.
The series resistance, DCR, within the inductor causes
conduction loss and heat dissipation. A shielded inductor
V
(
V
D
V
L
O
I
=
–
⁄
=
V
(
0 ) L ⁄
V
L
I
L
=
O
×
@ On = ΔI
1
ΔI
–
×
⁄
V
L
(
D
1 D
I
⁄
) V
Δt
×
–
⁄
t
S
O
)
=
(
L
V
O
@ Off, therefore:
SAT
–
V
) characteristics, power
D
14
–
V
I
)
⁄
L
×
1 (
D
–
D )
is Schottky diode
×
t
S
D
gives
(EQ. 6)
(EQ. 7)
(EQ. 8)
(EQ. 9)
ISL97672
is usually more suitable for EMI susceptible applications,
such as LED backlighting.
The peak current can be derived from the fact that the
voltage across the inductor during the Off period can be
shown as Equation 10:
The choice of 85% is just an average term for the
efficiency approximation. The first term is average current
that is inversely proportional to the input voltage. The
second term is inductor current change that is inversely
proportional to L and f
frequency and minimum input voltage the system
operates, the inductor I
given inductor size, usually the larger the inductance, the
higher the series resistance because of the extra winding
of the coil. Thus the higher the inductance, the lower the
peak current capability. The ISL97672 current limit may
also have to be taken into account.
Output Capacitors
The output capacitor acts to smooth the output voltage
and supplies load current directly during the conduction
phase of the power switch. Output ripple voltage consists
of the discharge of the output capacitor for I
FET On and the voltage drop due to flowing through the
ESR of the output capacitor. The ripple voltage can be
shown as Equation 11:
The conservation of charge principle in Equation 9 also
brings up a fact that during the boost switch Off period,
the output capacitor is charged with the inductor ripple
current minus a relatively small output current in boost
topology. As a result, the user needs to select an output
capacitor with low ESR and with a enough input ripple
current capability.
Output Ripple
ΔV
small ESR capacitors. In general, ceramic capacitors are
the best choice for output capacitors in small to medium
sized LCD backlight applications due to their cost, form
factor, and low ESR.
A larger output capacitor will also ease the driver
response during PWM dimming Off period due to the
longer sample and hold effect of the output drooping.
The driver does not need to boost harder in the next On
period that minimizes transient current. The output
capacitor is also needed for compensation, and in
general, 2x4.7µF/50V ceramic capacitors are suitable for
the notebook display backlight applications.
IL
ΔV
peak
Co
CO
can be reduced by increasing C
=
=
(
I (
V
O
O
⁄
×
C
I
O
O
×
)
⁄
D f
(
85%
⁄
S
)
S
+
×
. As a result, for a given switching
SAT
(
V
I (
I
)
O
must be chosen carefully. At a
+
×
1 2 V
ESR
⁄
[
)
I
×
(
O
V
O
or f
–
V
S
I
, or using
November 1, 2010
)
LPEAK
⁄
L (
×
(EQ. 11)
V
(EQ. 10)
during
FN7632.1
O
×
f
S
)
]