Rainbow Electronics MAX1797 - Manual (Page 10)

MAX1795/MAX1796/MAX1797
pate this energy, minimizing the ringing at LX.
Damping LX ringing does not reduce V
OUT
ripple, but
does reduce EMI (Figures 3, 4, and 5).
Setting the Output Voltage
V OUT can be set to 3.3V or 5.0V by connecting the FB
pin to GND (5V) or OUT (3.3V). To adjust the output
voltage, connect a resistive voltage-divider from OUT to
FB to GND (Figure 6). Choose a value less than 250k
Ω
for R2.
Use the following equation to calculate R1:
R1 = R2 [(V OUT / V
FB
) - 1]
where V FB = +1.245V, and V OUT may range from +2V
to +5.5V.
Low-Battery Detection
The MAX1795/MAX1796/MAX1797 each contain an on-
chip comparator for low-battery detection. If the voltage
at LBI is above 0.85V, LBO (an open-drain output)
sinks current to GND. If the voltage at LBI is below
0.85V, LBO goes high impedance. The LBI/LBO func-
tion remains active even when the part is in shutdown.
Connect a resistive voltage-divider to LBI from BATT to
GND. The low-battery monitor threshold is set by two
resistors, R3 and R4 (Figure 6). Since the LBI bias cur-
rent is typically 2nA, large resistor values (R4 up to
250k
Ω) can be used to minimize loading of the input
supply.
Calculate R3 using the following equation:
R3 = R4[(V
TRIP
/ 0.85V) - 1]
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
10 ______________________________________________________________________________________
Figure 3. Simplified Diagram of Inductor Damping Switch
MAX1795
MAX1796
MAX1797
DAMPING
SWITCH
BATT
R1
200
Ω
LX
OUT
22 µH
V IN
47 µF
V OUT
2 µs/div
V LX
1V/div
Figure 4. LX Ringing for Conventional Step-Up Converter
(without Damping Switch)
2 µs/div
V
LX
1V/div
Figure 5. LX Waveform with Damping Switch
Figure 6. Setting an Adjustable Output
GND
LBI
LBO
BATT
R3
R4
FB
LX
OUT
MAX1795
MAX1796
MAX1797
SHDN
47 µF 1M
OUTPUT
2V TO 5.5V
LOW-BATTERY
OUTPUT
R1
R2
47 µ F
V IN