Rainbow Electronics MAX773 - Manual (Page 15)

Determining R SENSE
The Typical Operating Characteristics graphs show the
output current capability for various modes, sense
resistors, and input/output voltages. Use these graphs,
along with the theoretical output current curves shown
in Figures 7a-7d, to select R
SENSE . These theoretical
curves assume that an external N-FET power switch is
used. They were derived using the minimum (worst-
case) current-limit comparator threshold value, and the
inductance value. No tolerance was included for
R
SENSE . The voltage drop across the diode was
assumed to be 0.5V, and the drop across the power
switch r
DS(ON) and coil resistance was assumed to be
0.3V. To use the graphs, locate the graph with the
appropriate output voltage or the graph having the
nearest output voltage higher than the desired output
voltage. On this graph, find the curve for the largest
sense-resistor value with an output current that is ade-
quate at the lowest input voltage.
Determining the Inductor (L)
Practical inductor values range from 10µH to 300µH.
20µH is a good choice for most applications. In appli-
cations with large input/output differentials, the IC’s
output current capability will be much less when the
inductance value is too low, because the IC will always
operate in discontinuous mode. If the inductor value
is too low, the current will ramp up to a high level
before the current-limit comparator can turn off the
switch. The minimum on-time for the switch (t
ON (min))
is approximately 2µs; select an inductor that allows
the current to ramp up to I
LIM /2 in no less than 2µs.
Choosing a value of I LIM /2 allows the half-size current
pulses to occur, increasing light-load efficiency and
minimizing output ripple.
MAX770–MAX773
5V/12V/15V or Adjustable, High-Efficiency,
Low I Q , Step-Up DC-DC Controllers
______________________________________________________________________________________ 15
MAXIMUM OUTPUT CURRENT (A)
0
INPUT VOLTAGE (V)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
2 3 4 5
R
SENSE
= 40m Ω
R SENSE = 50m Ω
R SENSE = 75m Ω
R SENSE = 200m Ω
R SENSE = 100m Ω
V OUT = 5V
L = 22 µ H
MAXIMUM OUTPUT CURRENT (A)
0
INPUT VOLTAGE (V)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
2 4 6 8 10 12
R SENSE = 200m Ω
R SENSE = 100m Ω
R SENSE = 40m Ω
R SENSE = 50m Ω
R SENSE = 75m Ω
V
OUT
= 12V
L = 22 µ H
Figure 7a. Maximum Output Current vs. Input Voltage
(V
OUT = 5V)
Figure 7b. Maximum Output Current vs. Input Voltage
(V
OUT = 12V)
Figure 7c. Maximum Output Current vs. Input Voltage
(V
OUT = 15V)
Figure 7d. Maximum Output Current vs. Input Voltage
(V
OUT = 24V)
MAXIMUM OUTPUT CURRENT (A)
0
INPUT VOLTAGE (V)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
2 4 6 8 10 12 14 16
R SENSE = 200m Ω
R SENSE = 100m Ω
V OUT = 15V
L = 22 µ H
R SENSE = 40m Ω
R SENSE = 50m Ω
R SENSE = 75m Ω
MAXIMUM OUTPUT CURRENT (A)
0
2
INPUT VOLTAGE (V)
0.8
6 10 14
0.2
0.4
0.6
V OUT = 24V
L =150 µ H
R
SENSE
= 100m Ω
R SENSE = 200m Ω
R SENSE = 400m Ω