Rainbow Electronics MAX776 - Manual (Page 13)

To choose the proper current-sense resistor, simply fol-
low the two-step procedure outlined below.
1) Determine:
• Input voltage range, V+
• Maximum (absolute) output voltage, V
OUT
• Maximum output current, I LOAD
For example, let V+ range from 4V to 6V, and choose
V
OUT = -24V and I OUT = 150mA.
2) Next, referring to Figure 9, find the curve with the
lowest current limit whose output current (with the
lowest input voltage) meets your requirements.
In our example, we want a curve where I OUT is >150mA
with a 4V input and a -24V output.
The R SENSE = 80m
Ω
(shown in Figure 9) shows only
approximately 125mA of output current with a 4V input,
so we look next at the R
SENSE = 70m
Ω
line. It shows
I OUT >150mA for V+ = 4V and V OUT = -24V. The cur-
rent limit will be 0.210V / 0.070
Ω
= 3A. These curves
take into account worst-case inductor (±10%) and
current-sense trip levels, but not sense-resistor toler-
ance. The switch on resistance is 70m
Ω
.
Standard wire-wound and metal-film resistors have
an inductance high enough to degrade performance.
Metal-film resistors are usually deposited on a ceramic
rod in a spiral, making their inductances relatively high.
Surface-mount (or chip) resistors have very little induc-
tance and are well suited for use as current-sense
resistors. If you want to use through-hole resistors, IRC
has a wire resistor that is simply a band of metal
shaped as a “U” so that inductance is less than 10nH
(an order of magnitude less than metal-film resistors).
These are available in resistance values between
5m
Ω
and 0.1
Ω
.
External Switching Transistor
The MAX774/MAX775/MAX776 are capable of driving
P-channel enhancement-mode MOSFET transistors only.
The choice of power transistor is dictated by input and
output voltage, peak current rating, on resistance, gate-
source threshold, and gate capacitance. The drain-to-
source rating must be greater than the V+ - V
OUT
input-to-output voltage differential. The gate-to-source
rating must be greater than V+ (the source voltage) plus
the absolute value of the most negative swing of EXT.
For bootstrapped operation, the most negative swing of
EXT is V
OUT . In non-bootstrapped operation, this may
be ground or some other negative voltage. Gate capac-
itance is not normally a limiting factor, but values should
be less than 1nF for best efficiency. For maximum effi-
ciency, the MOSFET should have a very low on resis-
tance at the peak current and be capable of handling
that current. The transistor chosen for the typical oper-
ating circuit has a 30V drain-source voltage limit and a
0.07
Ω drain-source on resistance at V GS = -10V.
Table 1 lists suppliers of switching transistors suitable
for use with the MAX774/MAX775/MAX776.
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
Inverting DC-DC Controllers
______________________________________________________________________________________ 13
Figure 8. MAX776 Maximum Output Current vs. Input Voltage
(V
OUT = -15V)
Figure 9. MAX774/MAX775/MAX776 Maximum Output Current
vs. Input Voltage (V
OUT = -24V)
MAXIMUM OUTPUT CURRENT (mA)
100
INPUT VOLTAGE (V)
MAX776-FIG08
200
300
400
500
600
700
3 4 5 6 7
R SENSE = 0.05 Ω
R SENSE = 0.06 Ω
R SENSE = 0.07 Ω
R SENSE = 0.08 Ω
R SENSE = 0.09 Ω
V OUT = -15V
MAXIMUM OUTPUT CURRENT (mA)
INPUT VOLTAGE (V)
0
3 4 5 6 7 8 9 10 11 12 13 14 15
MAX776-FIG09
200
400
600
800
R SENSE = 0.05 Ω
R SENSE = 0.06 Ω
R SENSE = 0.07 Ω
R SENSE = 0.08 Ω
R SENSE = 0.09 Ω
V OUT = -24V