LX1734
TM
®
1.0MHz Inverting DC/DC Converter
KEY FEATURES
Fixed Frequency 1.0MHz
Operation
Very Low Noise: 1mV
P-P
Output
Ripple Possible With Cuk
Topology
Stable Operation With Ceramic or
Tantalum Capacitors
-5V at 250mA from 5V Input
Uses Small Surface Mount L/C
Components
Wide Input Range: 4.2V to 8V
Low VCESAT Switch: 600mV at
600mA
6-Lead 3x3mm JEDEC MLPM
Package
Functionally Compatible with
LT1611 or LT1931
APPLICATIONS/BENEFITS
Disk Drive MR Head Bias
Digital Camera CCD Bias
LCD Bias
GaAs FET Bias
Local -5V or -12V Supplies
DESCRIPTION
The LX1734 is an inverting
DC/DC current-mode controller. With
a 750mA integrated switch, the
LX1734 can generate large output
currents in a small footprint. The
LX1734 minimizes external com-
ponent size and cost by implementing
a high switching frequency of
1.0MHz, while generating -5V at
250mA.
When configured in the dual
inductor inverting topology very low
output voltage ripple approaching
1mV
P-P
can be achieved when used in
conjunction with ceramic output
capacitors. The dual inductor can be
implemented as a coupled or separate
cores.
Fixed frequency operation ensures a
clean output free from low frequency
noise typically present with charge pump
solutions. The low impedance output
remains within 1% of nominal during
large load steps. The 18V switch allows
high voltage outputs to be generated.
The LX1734 is available in the space
saving 6-lead 3x3 Jedec MO-229
package, which has the same footprint
and lead spacing as the SOT-23A. A
complete inverter function utilizes less
than 0.3
2
inches of PCB space.
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IMPORTANT:
For the most current data, consult
MICROSEMI’s
website:
http://www.microsemi.com
PRODUCT HIGHLIGHT
L1A
22µH
V
IN
= 5V
V
IN
C1
10µF
SW
C2
1µF
L1B
22µH
R1
29.4k
LX1734
NFB
SD
GND
V
OUT
= -5V
@ 150mA
R2
10k
C4
1000pF
C3
22µF
Note: L1A and L1B are shown as coupled. Individual inductors can also be used.
C1, C2, C3 are ceramic capacitors
LX1734
LX1734
Figure 1
PACKAGE ORDER INFO
Plastic MLPL
LM
6-Pin
T (°C)
A
RoHS Compliant / Pb-free
Transition DC: 0452
0 to 85
LX1734CLM
Note: Available in Tape & Reel. Append the letters “TR” to the
part number. (i.e. LX1734CLM-TR)
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 1
LX1734
TM
®
1.0MHz Inverting DC/DC Converter
ABSOLUTE MAXIMUM RATINGS
PACKAGE PIN OUT
Supply Voltage (V
IN
), Shutdown (
SD
).................................................... 0 to 10V
SW Voltage....................................................................................... -0.4V to 20V
NFB Voltage .................................................................................................... -2V
Current Into NFB Pin................................................................................... ±1mA
Operating Temperature Range ............................................................0°C to 85°C
Maximum Junction Temperature ................................................................. 125°C
Storage Temperature......................................................................-65°C to 150°C
Peak Package Solder Reflow Temperature
(40 second maximum exposure) ..................................................... 260°C (+0, -5)
Note: Exceeding these ratings could cause damage to the device. All voltages are with respect to
Ground. Currents are positive into, negative out of specified terminal
.
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SW
GND
NFB
**
V
IN
N/C*
SD
LM P
ACKAGE
(Top View)
* Not Internally Connected.
** Package heatsink should
be connected to ground or
left floating.
RoHS / Pb-free 100% Matte Tin Lead Finish
THERMAL DATA
LM
Plastic LM 6-Pin
THERMAL RESISTANCE
-
JUNCTION TO
A
MBIENT
,
θ
JC
8°C/W
Junction Temperature Calculation: T
J
= T
A
+ (P
D
x
θ
JC
).
The
θ
JC
numbers are guidelines for the thermal performance of the device/pc-board
system. All of the above assume no ambient airflow.
FUNCTIONAL PIN DESCRIPTION
N
AME
SW
GND
Power Switch Pin
D
ESCRIPTION
Common ground reference
Feedback Pin - Connect to a resistive divider in order to set the output voltage. Feedback threshold is -1.235V.
Given the typical NFB bias current (I
NFB
) of 4µA flows out of the pin, the suggested value for R2 is 10K. Given
R
2
, set R
1
according to:
NFB
R
1
=
VOUT
1 . 235
R
2
−
1 . 235
+
I NFB
(
)
V
IN
Input Supply – Input pin must be locally bypassed.
Shutdown, Connected to >2V, device is active.
SD
P
ACKAGE
D
ATA
P
ACKAGE
D
ATA
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 2
LX1734
TM
®
1.0MHz Inverting DC/DC Converter
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, the following specifications apply over the operating ambient temperature 0°C
≤
T
A
≤
85°C and the following
test conditions: V
IN
= 5V
Parameter
Minimum Operating Voltage
V
IN
Under Voltage Lockout
Reference Voltage
Reference Voltage Line Regulation
NFB Pin Bias Current
Quiescent Current
Quiescent Current
Switching Frequency
Maximum Duty Cycle
Switch V
CESAT
Switch Leakage Current
Switch Circuit Current Limit
High
Low
Shutdown Input Voltage
Bias
Current
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Symbol
V
IN
UVLO
V
NFB
I
NFB
I
Q
I
SHDN
Test Conditions
Min
LX1734
Typ
Max
Units
V
V
V
mV
µA
mA
µA
MHz
%
mV
µA
mA
V
V
µA
µA
V
IN
rising, regulator remains off
4.5V
<
V
IN
<
5.5V, T
AMB
> 25°C
(Regulator Not Switching, V
NFB
= -2V)
V
< 0.28V
SD
I
OUT
= 5mA to 250mA
I
SW
= 600mA
V
SW
= 10V
Duty Cycle < 50%
Device Active
Device Disabled
V
SD
= 0.28V
V
SD
= 5V
V
SDH
V
SDL
I
SD
4.25
3.2
4.25
-1.205 -1.235 -1.255
18
-4
-8
9
12
300
0.8
1.4
82
650
800
0.02
1
700
2
0.8
-5
0.4
1.0
30
50
BLOCK DIAGRAM
V
IN
UVLO
BIAS
SD
SW
+
-
-
Q1
Q2
40pF
100k
+
Σ
R
SET
Q
S
CLR
Q
Q3
+
Ramp
Generator
1.1MHz
Oscillator
0.1Ω
-
E
LECTRICALS
E
LECTRICALS
NFB
R1 (External) R2 (External)
V
OUT
GND
C
PL
Figure 2
– Simplified Block Diagram
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 3
LX1734
TM
®
1.0MHz Inverting DC/DC Converter
CONDITIONS: V
IN
@ 5V, V
OUT
@ -5V, C
IN
=
COUT
=10uF Ceramic, L1=L2=10uH
STEP LOAD RESPONSE 0 – 100mA
STEP LOAD RESPONSE 0 – 250mA
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VOUT
VOUT
ISTEP LOAD
ISTEP LOAD
POWER ON RESPONSE, IOUT @ 200mA
OUTPUT VOLTAGE RIPPLE, IOUT @ 10mA and 150 mA
VOUT
IOUT = 10Ma
VIN
IOUT = 150mA
Inductor, 2A/div
LX1734 TEMPERATURE STABILITY
IL = 132mA
-4.84
-4.88
-4.92
-4.96
-5
-5.04
-5.08
-5.12
-5.16
-5.2
TEMP, °C
-15
Copyright
©
2002
Rev. 3.0a, 2005-03-14
Vout, Volts
FREQ, Khz
1070
1040
1010
980
950
920
890
860
830
800
90
105
120
Page 4
WAVEFORMS
WAVEFORMS
0
15
30
45
60
75
Microsemi
Microsemi Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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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