ISL97634(2007) 데이터 시트보기 (PDF) - Intersil
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ISL97634 Datasheet PDF : 10 Pages
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ISL97634
PART NO.
ISL97634IRT14Z
ISL97634IRT18Z
ISL97634IRT26Z
TABLE 1.
OVP
MAX NO. OF
LEDS
14V
3
18V
4
26V
6
MAX ILED
70mA
50mA
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 the following
formulas (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~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.
ILED = V-----F---B-----⋅---(---R----1-R----+2-----R⋅---R-2---S-)---–E----T-V----D----i--m-----⋅---R-----1-
(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).
These equations are valid for values of R1 and R2 such that
both R1>>RSET and R2>>RSET.
7
FN6264.2
April 10, 2007
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