ISL97634 データシートの表示（PDF） - Intersil
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ISL97634 Datasheet PDF : 10 Pages
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ISL97634
only rated for the required voltage range can therefore be
used, which will optimize the component costs in some cases.
TABLE 1.
PART NO.
OVP
MAX NO. OF
LEDS
MAX ILED
ISL97634IRT14Z
14V
3
70mA
ISL97634IRT18Z
18V
4
50mA
ISL97634IRT26Z
26V
7
30mA
Shutdown
When PWM/EN is taken low the ISL97634 enters into the
power-down mode where the supply current is reduced to
less than 1µA. The device resumes normal when the
PWM/EN goes high.
Components Selection
The input capacitance is typically 0.22µF. The output
capacitor should be in the range of 0.22µF to 1µF. X5R or
X7R type of ceramic capacitors of the appropriate voltage
rating are recommended.
When choosing an inductor, make sure the average and
peak current ratings are adequate by using Equations 3, 4 and
5 (80% efficiency assumed):
ILAVG
=
-I-L----E----D-----⋅---V----O-----U----T-
0.8 ⋅ VIN
(EQ. 3)
ILPK
=
IL
A
V
G
+
1--
2
⋅
ΔIL
(EQ. 4)
ΔIL
=
V-----I--N-----⋅---(--V-----O----U----T-----–----V-----I--N----)
L ⋅ VOUT ⋅ fOSC
(EQ. 5)
Where:
• ΔIL is the peak-to-peak inductor current ripple in Amps
• L is the inductance in H
• fOSC is the switching frequency, typically 1.45MHz
The ISL97634 supports a wide range of inductance values
(10µH to ~82µH). For lower inductor values or lighter loads,
the boost inductor current may become discontinuous. For
high boost inductor values, the boost inductor current will be
in continuous mode.
In addition to the inductor value and switching frequency, the
input voltage, number of LEDs and the LED current also
affects whether the converter operates in continuous
conduction or discontinuous conduction mode.
Both operating modes are allowed and normal. The
discontinuous conduction mode yields lower efficiency due
to higher peak current.
Compensation
The product of the output capacitor and the load create a
pole while the inductor creates a right half plane zero. Both
of these attributes degrade the phase margin but the
ISL97634 has internal compensation network that ensures
the device operates reliably under the specified conditions.
The internal compensation and the highly integrated
functions of the ISL97634 make it a design friendly device to
be used in high volume, high reliability applications.
Applications
Analog Dimming
Analog dimming is usually not recommended because of the
brightness non-linearity at low levels dimming. However,
some systems are EMI or noise sensitive that analog
dimming may be more suitable than PWM dimming under
those situations. The ISL97632 is part of the same family as
the ISL97634 and has been designed with a serial interface
to give access to 32 separate dimming levels. Alternatively
analog dimming can be achieved by applying a variable DC
voltage (VDim) at FB pin (see Figure 14) to adjust the LED
current. As the DC dimming signal voltage increases above
VFB, the voltages drop on R1 and R2 increase and the
voltage drop on RSET decreases. Thus, the LED current
decreases as shown in Equation 6:
ILED
=
V-----F---B-----⋅---(---R----1-----+-----R----2----)---–-----V----D----i--m-----⋅---R-----1-
R2 ⋅ RSET
(EQ. 6)
If VDIM is taken below FB, the inverse will happen and the
brightness will increase.
The DC dimming signal voltage can be a variable DC voltage
from a POT, a DCP (Digitally Controlled Potentiometer), or a
DC voltage generated by filtering a high frequency PWM
control signal.
L1
VIN
3.3V
C1
1µF
22µH
VIN
LX
VOUT
ISL97634
PWM
GND
FB
R2
LEDs
R1
3.3k
C2
0.22µF
RSET
4.75Ω
VDim
FIGURE 14. ANALOG DIMMING CONTROL APPLICATION
CIRCUIT
As brightness is directly proportional to LED currents, VDim
may be calculated for any desired “relative brightness” (F)
using Equation 7:
VDim
=
R-----2-
R1
⋅
VFB
⋅
⎛
⎜1
⎝
+
R-----1-
R2
–
⎞
F⎟
⎠
(EQ. 7)
Where F = ILED (dimmed)/ILED (undimmed).
7
FN6264.3
March 7, 2008
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