Datasheet

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, a curve where I
OUT
is >150mA with a
4V input and a -24V output is optimal.
The R
SENSE
= 80mΩ (Figure 9) shows only approxi-
mately 125mA of output current with a 4V input, so we
look next at the R
SENSE
= 70mΩ line. It shows I
OUT
>150mA for V+ = 4V and V
OUT
= -24V. The current limit
will be 0.210V / 0.070Ω = 3A. These curves take into
account worst-case inductor (±10%) and current-
sense trip levels, but not sense-resistor tolerance. The
switch on resistance is 70mΩ.
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. 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 mag-
nitude less than metal-film resistors). These are avail-
able in resistance values between 5mΩ and 0.1Ω.
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 nonbootstrapped operation, this may be
ground or some other negative voltage. Gate capaci-
tance 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 operat-
ing circuit has a 30V drain-source voltage limit and a
0.07Ω 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-to-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
34567
R
SENSE
= 0.05Ω
R
SENSE
= 0.06Ω
R
SENSE
= 0.07Ω
R
SENSE
= 0.08Ω
R
SENSE
= 0.09Ω
V
OUT
= -15V
MAXIMUM OUTPUT CURRENT (mA)
INPUT VOLTAGE (V)
0
345678 910111213 14 15
MAX776-FIG09
200
400
600
800
R
SENSE
= 0.05Ω
R
SENSE
= 0.06Ω
R
SENSE
= 0.07Ω
R
SENSE
= 0.08Ω
R
SENSE
= 0.09Ω
V
OUT
= -24V