Rainbow Electronics MAX8514 User Manual
Page 29
MAX8513/MAX8514
Wide-Input, High-Frequency, Triple-Output Supplies
with Voltage Monitor and Power-On Reset
______________________________________________________________________________________
29
the package’s power dissipation could limit the useable
maximum input-to-output voltage differential.
The maximum power-dissipation capability of the tran-
sistor’s package and mounting must allow the actual
power dissipation in the device without exceeding the
maximum junction temperature. The power dissipated
equals the maximum load current multiplied by the
maximum input-to-output voltage differential.
When the MAX8513/MAX8514 are disabled, R26 dis-
charges C7.
OUT3P Voltage Selection (PNP)
The MAX8513 positive linear-regulator output voltage,
V
OUT3P
, is set with a resistive-divider from OUT3P to
FB3P to GND. First, select R14 resistance value (below
1kΩ). Then, solve for R13 so:
where V
OUT3P
can range from +0.8V to +27V.
OUT3N Voltage Selection (NPN)
The MAX8514’s negative linear-regulator output volt-
age, V
OUT3N
, is a negative regulated voltage devel-
oped through the pass transistor Q4 (MAX8514 Typical
Applications Circuits). A resistive-divider from OUT3N
to FB3N to V
REF3N
forces V
FB3N
to regulate to 0V.
Calculate V
OUT3N
by first selecting R14 the resistor
from V
REF3N
to FB3N to be below 5kΩ, where V
REF3N
is any positive voltage (usually V
OUT1
). R13 is then cal-
culated by:
SUP3N is the supply input for OUT3N’s transconduc-
tance amplifier. When OUT3N is used, SUP3N must be
connected to a voltage supply between 1.5V and 5.5V
that can source at least 25mA. Typically, V
OUT1
can be
used as the supply input for SUP3N.
Stability Requirements
The MAX8513/MAX8514s’ DRV3P and DRV3N outputs
are designed to drive bipolar transistors (PNP types for
the MAX8513 with the DRV3P output, and NPN types
for the MAX8514 with the DRV3N output). These bipolar
transistors form linear regulators with positive outputs
(MAX8513 from 0.8V to 27V) and negative outputs
(MAX8514 from -18V to -1V). An internal transconduc-
tance amplifier is used to drive the external pass tran-
sistors. The transconductance amplifier, pass
transistor’s specifications, the base-emitter resistor,
and the output capacitor determine the loop stability.
The total DC loop gain (A
V
) is the product of the gains
of the internal transconductance amplifier, the gain
from base to collector of the pass transistor (Q4 in the
Typical Applications Circuits), and the gain of the feed-
back divider.
The transconductance amplifier regulates the output
voltage by controlling the pass transistor’s base cur-
rent. Its DC gain is approximately:
where G
C3_
is typically 0.6S (OUT3P) and 0.36S
(OUT3N), R
IN
is the input resistance of Q4, and can be
calculated by:
The DC gain for the transistor (Q4), including the feed-
back divider, is approximately:
V
T
is the thermal voltage for the transistor (typically 26mV
at T
A
= +27°C). The total DC loop gain for OUT3_ is:
A dominant pole (f
POLE1
) is created from the output
capacitance and load resistance:
Unity-gain crossover (f
C_OUT3_
) should occur at:
A second pole is set by the input capacitance to the
base of Q4 (C
Q4IN
), any external base-to-emitter
capacitance (C
BE
, see the Base-Drive Noise Reduction
section and Figure 7), the transistor’s input resistance
(R
IN
), and the base-to-emitter pullup resistor (R12):
f
A
f
C OUT
V
POLE
_
_
3
1
=
×
f
C
R
I
C
V
POLE
OUT
OUT
OUT
MAX
OUT
OUT
1
3
3
3
3
3
1
2
2
=
Ч
Ч
=
Ч
Ч
π
π
_
_
A
G
R
R
A
V
C
IN
Q
=
Ч
(
)
Ч
3
4
12
_
_
||
A
V
VT
for OUT P or
A
V
V
V
V
V
Q P
REF
Q N
OUT N
REF N
REF N
OUT N
T
4
4
3
3
3
3
3
=
=
Ч
(
)
Ч
-
R
mV
I
IN
OUT
_
=
26
3
β
G
R
R
C
IN
3
12
_
||
×
R
V
V
R
OUT N
REF N
13
14
3
3
=
×
-
R
R
V
V
OUT P
13
14
0 8
3
.
=
-1