LX1734
TM
®
1.0MHz Inverting DC/DC Converter
THEORY OF OPERATION
The LX1734 is a fixed frequency current mode controller
designed to develop a negative output voltage from a positive
input voltage. The switching transistor and current sense resistor
are integrated into the part. The PWM functions in a peak current
regulation mode using the amplified error signal to determine the
peak switch current each cycle. Slope compensation is added to
provide stable operation at high duty cycles. A current limit
detector overrides the regulation loop and prevents the switch
current from exceeding the over current threshold level.
The bandgap control circuit keeps Q1 biased on and produces a
reference current (I
REF
) that produces a voltage drop across the
internal resistance that has a positive temperature coefficient.
When this resistor voltage drop is added to the negative temperature
coefficient of the base-emitter voltage drop of Q1, the result is a
temperature compensated reference voltage (V
REF
) at the NFB pin.
The summing node from the external feedback network is
connected directly to NFB pin, which is relatively high impedance
(typically 150k). The feedback loop minimizes the error current,
(I
ERROR
) which effectively regulates the voltage at the NFB pin. As
with a conventional error amplifier, the error signal is proportional
to the difference between the temperature compensated reference
voltage (V
REF
) and the summing node voltage. A slight correction
factor is necessary to account for the added summing node voltage
due to the reference current (I
REF
, typically 4µADC) flowing
through the Thevenin equivalent summing node external resistance.
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APPLICATION NOTE
The LX1734 can be used in several topologies that generate a
negative output voltage from a positive input voltage. The
LX1734 can be used in a dual inductor converter with coupled or
uncoupled inductors (see Figure 1); this topology is required if
the absolute value of the output voltage is less than or equal to
the input voltage but can also be used for higher voltage outputs.
The following components or their equivalents can be used to
implement the converter in Figure 1, which produces a –5V
output at 150mA from a +5V input. The reference design has an
efficiency of greater than 72% and an input ripple voltage of less
than 6mV
P-P
and an output ripple voltage of less than 300µV
P-P
.
Ref
C1
C2
C3
C4
D1
L1
Description
Ceramic, 4.7uF,
6.3V (0805)
Ceramic, 1uF, 16V
(0805)
Ceramic, 22uF,
6.3V (1210)
Ceramic, 470pF,
50V (0402)
Diode, 0.5A, 30V
Inductor, Coupled,
22uH
Part Number
JMK212BJ475MG
GRM40X7R105M16
JMK325BJ226MM
GRM36X7R471K050
UPS530
CLS62-220NC
Manufacturer
Taiyo Yuden
Murata
Taiyo Yuden
Murata
Microsemi
Sumida
Inductor Selection
When the LX1734 is used in a dual inductor converter with coupled
inductors, a parallel winding inductor value of 22µH works well for
a 5V input and a -5V output at 150mA. The inductor value can be
scaled to the particular set of operating conditions based on the
input voltage, output voltage, and output current. The new value of
coupled inductor parallel inductance can be calculated using the
following equation:
⎛
V
⎞ ⎛
150 mA
⎞ ⎛
−
5 V
⎞
⎟
×
⎜
⎟
L
NEW
=
22 µH
×
⎜
IN
⎟
×
⎜
⎝
5 V
⎠ ⎜
I
OUT
⎟ ⎜
V
OUT
⎟
⎝
⎠ ⎝
⎠
The inductor value should be rounded to the nearest available value.
The parallel saturation current rating of a coupled inductor should
be sized to carry the summation of the peak input and peak output
inductor currents.
When the LX1734 is used in a dual inductor converter with two
separate (uncoupled) inductors or when using the boost converter
with an inverting charge pump output configuration, the inductance
value for each inductor should be about twice the value
recommended for a coupled inductor.
The peak current in the inductor is the DC current plus �½ of the
peak-to-peak ripple current. The saturation current rating of the
inductors should be sized to carry the peak inductor current. The
peak-to-peak ripple current can be calculated based on the inductor
value, the terminal voltage (input or output), and the duty cycle.
The DC inductor current is the same as the DC output current on the
output inductor. The DC input current includes the power for the
LX1734, but is still a good approximation for the DC inductor
current for higher power applications.
For simplicity, the
calculations below ignore the voltage drops of the switch and diode.
The duty cycle, D, for the dual inductor topology (assuming
continuous inductor current mode operation) is approximately:
Table 1
- Part List for Figure 1 (All Parts Are Surface Mount).
Separate inductors (not on a common core) can be used in place
of the coupled inductor (L1) of Figure 1. In this case the only
component that changes in the parts list is L1, which now would
be two separate inductors (L1, formerly L1A, and L2, formerly
L1B). With the separate inductors the peak-to-peak voltage
ripple on the input the output were less the 2mV
P-P
and less than
500µV
P-P
, respectively.
Table 2
- Part List For Alternative Inductors
Ref.
Designator
L1, L2
Description
Inductor, 47uH,
(1812)
Part Number
LQH4C470K04M00
Manufacturer
Murata
A
PPLICATION
A
PPLICATION
Copyright
©
2002
Rev. 3.0a, 2005-03-14
Microsemi
Microsemi Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
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