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ISL97672B
Low-Voltage Operations
The ISL97672B VIN pin can be separately biased from the LED
power input to allow low-voltage operation. For systems that have
only a single supply, V OUT can be tied to the driver VIN pin to allow
initial start-up (Figure 23). The circuit works as follows: when the
V IN = 2.7V~26.5V
Q1 OPTIONAL
ISL97672B
V OUT = 45V*, 6 x 50mA*
input voltage is available and the device is not enabled, V OUT
follows V IN with a Schottky diode voltage drop. The V OUT
boot-strapped to the VIN pin allows initial start-up, once the part
is enabled. Once the driver starts up with V OUT regulating to the
target, the VIN pin voltage also increases. As long as V OUT does
not exceed 26.5V and the extra power loss on V IN is acceptable,
this configuration can be used for input voltage as low as 3.0V.
The Fault Protection FET feature cannot be used in this
configuration.
V BIAS = 5V~12V
1 FAULT
2 VIN
4 VDC
6 /FLAG
5 PWM
LX 20
OVP 16
PGND 19
CH0 10
CH1 11
CH2 12
For systems that have dual supplies, the VIN pin can be biased
from 5V to 12V, while input voltage can be as low as 2.7V
(Figure 24). In this configuration, V BIAS must be greater than or
equal to V IN to use the fault FET.
V OUT = 26.5, 6 x 50mA*
V IN = 3.0V~21V
3 EN
17 RSET
8 FSW
9 AGND
CH3 13
CH4 14
CH5 15
COMP 18
* V IN > 12V
FIGURE 24. DUAL SUPPLY 2.7V OPERATION
ISL97672B
Compensation
1 FAULT
2 VIN
4 VDC
6 /FLAG
5 PWM
3 EN
17 RSET
8 FSW
9 AGND
LX 20
OVP 16
PGND 19
CH0 10
CH1 11
CH2 12
CH3 13
CH4 14
CH5 15
COMP 18
* V IN > 12V
The ISL97672B incorporates a transconductance amplifier in its
feedback path to allow the user to optimize boost stability and
transient response. The ISL97672B uses current mode control
architecture, which has a fast current sense loop and a slow
voltage feedback loop. The fast current feedback loop does not
require any compensation, but for stable operation, the slow
voltage loop must be compensated. The compensation is a
series of Rc, Cc1 network from COMP pin to ground, with an
optional Cc2 capacitor connected between the COMP pin and
ground. The Rc sets the high-frequency integrator gain for fast
transient response, and the Cc1 sets the integrator zero to
ensure loop stability. For most applications, the component
values in Figure 25 can be used: Rc is 10k ? and Cc1 is 3.3nF.
Depending upon the PCB layout, for stability, a Cc2 of 390pF may
be needed to create a pole to cancel the output capacitor ESR’s
zero effect.
FIGURE 23. SINGLE SUPPLY 3.0V OPERATION
Rc 10k
COMP
Cc1
3.3nF
Cc2
390pF
FIGURE 25. COMPENSATION CIRCUIT
14
FN7995.1
November 22, 2013
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