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MAX774CPA View Datasheet(PDF) - Maxim Integrated

Part NameDescriptionManufacturer
MAX774CPA -15V or adjustable, high-efficiency, low IQ inverting DC-DC controller MaximIC
Maxim Integrated MaximIC
MAX774CPA Datasheet PDF : 16 Pages
First Prev 11 12 13 14 15 16
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
RSENSE = 0.05
RSENSE = 0.06
RSENSE = 0.07
VOUT = -15V
RSENSE = 0.08
RSENSE = 0.09
Figure 8. MAX776 Maximum Output Current vs. Input Voltage
(VOUT = -15V)
VOUT = -24V
RSENSE = 0.05
600 RSENSE = 0.06
RSENSE = 0.07
RSENSE = 0.08
RSENSE = 0.09
5 6 7 8 9 10 11 12 13 14 15
Figure 9. MAX774/MAX775/MAX776 Maximum Output Current
vs. Input Voltage (VOUT = -24V)
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, VOUT
• Maximum output current, ILOAD
For example, let V+ range from 4V to 6V, and choose
VOUT = -24V and IOUT = 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 IOUT is >150mA
with a 4V input and a -24V output.
The RSENSE = 80m(shown in Figure 9) shows only
approximately 125mA of output current with a 4V input,
so we look next at the RSENSE = 70mline. It shows
IOUT >150mA for V+ = 4V and VOUT = -24V. The cur-
rent limit will be 0.210V / 0.070= 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.
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
5mand 0.1.
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+ - VOUT
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 VOUT. 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
0.07drain-source on resistance at VGS = -10V.
Table 1 lists suppliers of switching transistors suitable
for use with the MAX774/MAX775/MAX776.
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