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ISL97691
1.35 × 24.53 × ( 4 × 0.03 )
I L = ----------------------------------------------------------------- = 1.47A
( 8.65 × 10
)
f SW = --------------------------------
Boost Output Voltage Range
The working range of the boost output voltage, V OUT is from 40%
to 100% of the maximum output voltage, V OVP , set by resistors
R 1 and R 2 as described in the previous section.
The target applications should be considered carefully to ensure that
V OVP is not set unnecessarily high. For example, using R 1 = 470k ?
and R 2 = 23.7k ? per the Typical Application Circuit on page 2 sets
V OVP to between 24.53V to 25.88V when tolerancing is considered.
The minimum voltage, V OVP (min) = 24.53V, sets the maximum
number of LEDs per channel because this the worst case
minimum voltage that the boost converter is guaranteed to supply.
The maximum voltage, V OVP (max) = 25.88V, sets the minimum
number of LEDs per channel because it sets the lowest voltage
that the boost converter is guaranteed to reach: 40% x
25.88V = 10.35V.
Using LEDs with a V F tolerance of 3V to 4V, this V OVP example is
suitable for strings of 4 to 6 LEDs. If fewer than 4 LEDs per
channel are specified, V OVP must be reduced.
Switching Frequency
The boost switching frequency is adjusted by resistor R FSW
(Equation 11):
10
(EQ. 11)
R FSW
Where:
f SW is the desirable boost switching frequency (Hz)
R FSW is resistor from FSW pin to GND ( ? )
Inductor
Choose the inductance according to Table 1:
TABLE 1. INDUCTOR SELECTION
For example, for a system using 4 LED channels with 30mA per
channel and a maximum output voltage (OVP) of 24.53V with an
input supply of 2.7V minimum:
(EQ. 13)
2.7
Output Capacitor
It is recommended that a 2.2μF to 3.3μF X5R/X7R or equivalent
ceramic output capacitor is used.
Schottky Diode
The Schottky diode should be rated for at least the same forward
current as the inductor, and for reverse voltage to at least the
maximum output voltage, OVP.
Compensation
The ISL97691’s boost regulator uses a current mode control
architecture with a standardised external compensation network
connected to the COMP pin. The component values shown in the
Typical Application Circuit, Figure 1, on page 1 should be used.
The network comprises a series RC of 12k ? and 15nF also from
COMP to GND.
Applications
Unused LED Channels
Connect unused LED channels to GND.
High Current Applications
Each channel of the ISL97691 supports 40mA continuous sink
current. For applications that need higher current, multiple
channels can be paralleled (Table 2).
TABLE 2. PARALLELING CHANNELS FOR HIGHER CURRENT
BOOST FREQUENCY
400kHz to 700kHz
700kHz to 1MHz
1MHz to 1.5MHz
INDUCTANCE
10μH to 15μH
6.8μH to 10μH
4.7μH to 8.2μH
TOTAL
CHANNELS
4
2
1
CHANNEL
CURRENT
40mA per channel
80mA per channel
160mA
CHANNEL CONNECTIONS
CH1, CH2, CH3, CH4
{CH1 & CH2}, {CH3 & CH4}
{CH1 & CH2 & CH3 & CH4}
1.5MHz
3.3μH to 4.7μH
NOTE: PWMI_1 and PWMI_2 must driven together for total channels 1.
The inductor saturation current rating should be at least the
figure provided by Equation 12:
The example below shows CH1 & CH2 paralleled.
I L = -----------------------------------------------------
1.35 × V OUT × I LED
V IN
Where:
(EQ. 12)
I L is the minimum inductor saturation current rating (A)
V OUT is the maximum output voltage set by OVP (V)
I LED is the sum of the channel currents (A)
VIN is the minimum input voltage (V)
If the calculation produces a current rating higher than the 3.15A
maximum boost switch current limit, then a 3A inductor current
rating is adequate.
14
CH1
CH2
FIGURE 22.
FN7840.1
February 1, 2013
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