CAT3661 Datasheet, PDF (10/12 Pages) ON Semiconductor – 1-Channel Low-Power LED Driver

CAT3661
range. Extra external resistors can be added to the top or
bottom of the internal resistor divider network to alter the
divider ratio gain factor. The low battery indicator trip point
can be calculated by the following formula:
V_LB + V_ADJ G_ADJ
V_LB = Low Battery Voltage Trip Point
V_ADJ = Low Battery Comparator Trip point (0.6 V)
G_ADJ = Resistor Divider Gain (4 internally)
To obtain a low battery trip point of 2.4 V, the ADJH pin
is shorted to VIN, and the ADJL pin is tied to GND.
To increase the low battery trip point, insert a resistor
between ADJH and VIN. To consequently lower the low
battery trip point, insert a resistor between ADJL and GND.
The following formula shows how to calculate the modified
resistor divider gain:
G_ADJM
+
R_ADJ ) R_H
(R_ADJńG_ADJ) )
R_L
G_ADJM = Modified resistor divider gain
R_ADJ = Total resistance of divider (800 kW typ.)
R_H = High external resistor (ADJH to VIN)
R_L = Low external resistor (ADHL to GND)
Figure 25. Application Circuit with R_H & R_L
The resistance required for a certain trip point voltage can
be calculated by rearranging the above equations with
respect to R_H or R_L.
For V_LB > 2.4 V, use R_L = 0 W and
R_H (kW) ≅ 200 (V_LB/0.6 – 1) − 600
For V_LB < 2.4 V, use R_H = 0 W and
R_L (kW) ≅ 600 (0.6/(V_LB – 0.6)) – 200
Figure 26 shows the external resistor value for low battery
voltage trip points (V_LB) between 2 V and 3.2 V. For V_LB
above 2.4 V, R_L = 0 W. For V_LB below 2.4 V, R_H = 0 W.
240
200
160
120
RH
80
RL
40
0
2.0
2.2
2.4
2.6
2.8
TRIP POINT VOLTAGE (V)
3.0 3.2
Figure 26. V_LB vs. R_H & R_L
The low battery trip point does not operate for adjustments
below 2.0 V VIN.
The inclusion of the ADJH pin allows monitoring of
supplies other than the supply to the CAT3661. Simply
connect ADJH pin directly to the supply to be monitored and
the low battery indicator will function as normal when the
device is enabled. When EN is low, no current will flow in
the resistor divider network allowing ‘zero’ current
shutdown mode.
Under Voltage Lockout
If the voltage on VIN is less than V_UVLO threshold, the
nFLTB output is driven low and the device enters a low
power state where the LED output is off.
When the device is in shutdown (EN low), the nFLTB pin
will float high to ‘zero’ current state.
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