MAX1965TEEP Maxim Integrated, MAX1965TEEP Datasheet - Page 21

• Manufacturer Part Number: MAX1965TEEP
• Description: Current & Power Monitors & Regulators
• Manufacturer: Maxim Integrated

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A signal diode, such as the 1N4148, works well in most
applications. If the input voltage goes below 6V, use a
small 20mA Schottky diode for slightly improved effi-
ciency and dropout characteristics. Do not use large
power diodes, such as the 1N5817 or 1N4001, since
high junction capacitance can charge up VL to exces-
sive voltages.
The MAX1964/MAX1965’s positive linear regulator out-
put voltages are set by connecting a voltage divider
from the output to FB_ to GND (Figure 6). Select R4 in
the 1kΩ to 50kΩ range. Calculate R3 with the following
equation:
where V
30V.
The MAX1965’s negative output voltage is set by con-
necting a voltage divider from the output to FB5 to a
positive voltage reference (Figure 6). Select R6 in the
1kΩ to 50kΩ range. Calculate R5 with the following
equation:
where V
V
If the negative regulator is used, the OUT pin must be
connected to a voltage supply between 2V and 5V that
OUT
With the output pole’s frequency and series com-
pensation capacitor values, the required series
resistance can be determined. Based on the above
equation, select R
Now we must determine if the selected output
capacitor’s ESR generates a second zero before
crossover—the circuit shown in Figure 1 uses a
1000µF 10V Sanyo CZ-series electrolytic capacitor
with an ESR rating of 0.2Ω, so the zero occurs at
800Hz. Since crossover occurs at 40kHz, add the
second parallel compensation capacitor.
Finally, the second compensation capacitor value
must be approximately 43pF. Select the closest
standard capacitor value of 47pF.
may be set between 0 and -20V.
Negative Output Voltage Selection (MAX1965)
FB
REF
= 1.24V and V
is the positive reference voltage used and
R
______________________________________________________________________________________
Linear Regulator Controllers
3
Positive Output Voltage Selection
R
=
5
COMP
R
=
4
R




Tracking/Sequencing Triple/Quintuple



OUT
6
V



V
= 5.1MΩ.
OUT
V
V
FB
OUT
REF
may range from 1.24V to



Boost-Supply Diode
-



1




Power-Supply Controllers
can source at least 25mA. Typically, the OUT pin is
connected to the step-down converter’s output.
However, if the step-down converter’s output voltage is
set higher than 5V, OUT must be connected to one of
the positive linear regulators with an output voltage
between 2V and 5V.
The pass transistors must meet specifications for cur-
rent gain (h
uration voltage, and power dissipation. The transistor’s
current gain limits the guaranteed maximum output cur-
rent to:
where I
and R
between the transistor’s base and emitter. Furthermore,
the transistor’s current gain increases the linear regula-
tor’s DC loop gain (see Stability Requirements), so
excessive gain will destabilize the output. Therefore,
transistors with current gain over 100 at the maximum
output current, such as Darlington transistors, are not
recommended. The transistor’s input capacitance and
input resistance also create a second pole, which
could be low enough to destabilize the output when
heavily loaded.
The transistor’s saturation voltage at the maximum out-
put current determines the minimum input-to-output
voltage differential that the linear regulator will support.
Alternatively, the package’s power dissipation could
limit the useable maximum input-to-output voltage dif-
ferential. The maximum power dissipation capability of
the transistor’s package and mounting must exceed the
actual power dissipation in the device. The power dissi-
pated equals the maximum load current times the maxi-
mum input-to-output voltage differential:
The MAX1964/MAX1965 linear regulators use an inter-
nal transconductance amplifier to drive an external
pass transistor. The transconductance amplifier, pass
transistor’s specifications, the base-emitter resistor,
and the output capacitor determine the loop stability. If
the output capacitor and pass transistor are not proper-
ly selected, the linear regulator will be unstable.
The transconductance amplifier regulates the output
voltage by controlling the pass transistor’s base cur-
rent. Since the output voltage is a function of the load
P = I
BE
LOAD(MAX)
DRV
I
LOAD MAX
(220Ω) is the pullup resistor connected
FE
is the minimum 10mA base drive current
), input capacitance, collector-emitter sat-
(
(V
)
LDOIN -
=




I
DRV
V
-
OUT



R
Stability Requirements
V
BE
BE
) = I
Transistor Selection







LOAD(MAX)
h
FE MIN
(
)
V
CE
21