6
FN9239.3
March 22, 2010
Detailed Description
The ISL97632 uses a constant frequency, current mode
control scheme to provide excellent line and load regulation.
There are three OVP models for driving 3, 4, and 7 LEDs
(3.5V/20mA type) and their OVP thresholds
are set at 14V,
18V, and 26V respectively. The ISL97632 operates from an
input voltage of 2.4V to 5.5V and ambient temperature
from -40°C to +85°C. The switching frequency is around
1.45MHz and allows the driver circuit to employ small LC
components. The forward current of the LED is set using the
R
SET
resistor. In the steady state mode, the LED current is
given by Equation 1:
where S is the 5-bit Serial Interface Setting or Digital code
from 0 to 31 programmed in the XSD single-wire interface.
The default setting is 0 and the VFB is at minimum.
Dimming Control
The ISL97632 powers up to provide minimium current. By
programming the digital code with the Intersil’s 1-Wire XSD
interface as shown in Figure 1, the current can be changed
linearly with the digital code from 0 to 31. Figure 6 shows
LED current versus the programming codes.
Overvoltage Protection
The ISL97632 comes with overvoltage protection. The
OVP trip points are at 14V, 18V, and 26V for
ISL97632IRT14Z, ISL97632IRT18Z, and
ISL97632IRT26Z respectively. The maximum numbers of
LEDs and OVP threshold are shown in Table 1. When the
device reaches the OVP, the LX stops switching, disabling
the boost circuit until V
OUT
falls about 7% below the OVP
threshold. At this point, LX will be allowed to switch again.
The OVP event will not cause the device to shutdown
.
There are three OVP options. The 3 LEDs application
should use the 14V OVP device. The 7 LEDs application
should use the 26V OVP device. An output capacitor that is
only rated for the required voltage range can therefore be
used which will optimize the component costs in some
cases.
Shut-Down
An active high EN pin is normally on but this pin can be used
as a shutdown power saving function or zero brightness
setting. When taken low the EN pin places the ISL97632 into
power down mode down where the supply current is
reduced to less than 1µA. The EN pin cannot be used as
PWM input, as the part resets to 0 whenever EN is low. To
resume previous setting, the device needs to be
reprogrammed.
Output Disconnect
The ISL97632 features a FBSW feedback disconnect switch
that can be used in between the LED and R
SET
for an
optional short-circuit protection. For example, the user may
build an external short circuit detection to monitor the V
OUT
.
If the V
OUT
goes low due to one or more LEDs which are
shorted, the circuit can release the EN and FBSW switch to
disconnect the LEDs.
Components Selection
The input capacitance is typically 0.22µF to 4.7µF. 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 2, 3
and 4 (80% efficiency assumed):
Where:
• I
L
is the peak-to-peak inductor current ripple in Amps
• L is the inductance in H.
•f
OSC
is the switching frequency, typically 1.45MHz
The ISL97632 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, the number of LEDs and the LED current also
affect whether the converter operates in continuous
conduction or discontinuous conduction mode. Both operating
modes are allowed and normal. The discontinuos 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
attributes degrade the phase margin but the ISL97632 has
an internal compensation network that ensures the device
operates reliabily under the specified conditions. The
internal compensation and the highly integrated functions of
the ISL97632 make it a design friendly device to be used in
high volume high reliability applications.
TABLE 1.
PART NO. OVP
MAX NO. OF
LEDS MAX ILED
ISL97632IRT14Z 14V 3 70mA
ISL97632IRT18Z 18V 4 50mA
ISL97632IRT26Z 26V 7 30mA
I
LED
S
V
FB
S
R
SET
--------------------
9.8mV 5.68mV S+
R
SET
--------------------------------------------------------
==
(EQ. 1)
I
LAVG
I
LED
V
OUT
0.8 V
IN
---------------------------------
=
(EQ. 2)
I
LPK
I
LAVG
1
2
---
I
L
+=
(EQ. 3)
I
L
V
IN
V
OUT
V
IN
–
LV
OUT
f
OSC
---------------------------------------------------
=
(EQ. 4)
ISL97632
