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MAX845ESA+ |MAX845ESAMAXIMN/a90avaiIsolated Transformer Driver for PCMCIA Applications
MAX845ESA+T |MAX845ESATMAXIMN/a7000avaiIsolated Transformer Driver for PCMCIA Applications
MAX845EUA+ |MAX845EUAMAXIMN/a4100avaiIsolated Transformer Driver for PCMCIA Applications
MAX845EUA+T |MAX845EUATMAXIMN/a10671avaiIsolated Transformer Driver for PCMCIA Applications


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MAX845ESA+-MAX845ESA+T-MAX845EUA+-MAX845EUA+T
Isolated Transformer Driver for PCMCIA Applications
_______________General Description
The MAX845 provides an isolated power supply small
enough to fit in thin PCMCIA cards and space-sensitive
applications. It drives a low-profile center-tapped trans-
former primary from a 5V or 3.3V DC power supply. The
secondary can be wound to provide any isolated posi-
tive or negative voltage at powers up to 750mW.
The MAX845 consists of an oscillator followed by a tog-
gle flip-flop. The flip-flop generates two 50% duty-cycle
square waves, which are complementary at half the
oscillator frequency (450kHz, min). These two signals
drive the ground-referenced N-channel power switch-
es. Internal circuitry ensures break-before-make action
between the two switches.
A low-power shutdown disables both the switches and
the oscillator, reducing power consumption. An evalua-
tion kit (MAX845EVKIT-MM) is available to evaluate low-
profile 5V 40mA and 5V 100mA applications.
________________________Applications

PCMCIA Modem Cards
Isolated Data Acquisition
Isolated Interface Power Supply
Noise-Immunity Communications Interface
Bridging Ground Differences
Medical Equipment
Process Control
Low-Power LAN Networks
____________________________Features
Transformer Driver for Ultra-Thin 5V-μs Transformers Isolated DC-to-DC Power Supply for PCMCIA
Applications
450kHz Minimum Switching FrequencyUltra-Low Input Supply Current RippleSingle +5V or +3.3V Supply5μW Low-Power Shutdown Mode8-Pin SO and μMAX PackagesLow Output Ripple Permits Miniature Output
Capacitorssolated Transformer Driver
for PCMCIA Applications

GND2
VCC
N.C.SD
GND1
SO/μMAX

TOP VIEW
MAX845
___________________Pin Configuration

MAX845FS
GND1GND2
VCC167
VIN
FREQUENCY
SELECT
5V @ 150mA
OUTPUTON / OFF
CR2
CR1
__________Typical Operating Circuit

19-0372; Rev 4; 10/97
PART

MAX845C/D
MAX845EUA-40°C to +85°C
0°C to +70°C
TEMP. RANGEPIN-PACKAGE

Dice*
8 μMAX
EVALUATION KIT
AVAILABLE

*Contact factory for dice specifications.
_______________Ordering Information

MAX845ESA-40°C to +85°C8 SO
solated Transformer Driver for PCMCIA Applications
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS

(VCC= 5V ±10%, TA= TMINto TMAX, unless otherwise noted. Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Note 1:
Operating supply current is the current used by the MAX845 only. Load current is not included.
Note 2:
Shutdown supply current includes output switch leakage currents.
Supply Voltage (VCC)...............................................-0.3V to +7V
Control Input Voltage (SD, FS)...................-0.3V to (VCC+ 0.3V)
Peak Output Switch Current (D1, D2)......................................1A
Output Switch Voltage (D1, D2).............................................12V
Average Output Switch Current (D1, D2).........................200mA
Continuous Power Dissipation (TA= +70°C)
SO (derate 5.88mW/°C above +70°C).........................471mW
μMAX (derate 4.10mW/°C above +70°C)....................330mW
Operating Temperature Range...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +160°C
Junction Temperature......................................................+150°C
Lead Temperature (soldering, 10sec).............................+300°C
FS = VCC = 5.5V
FS = VCC= 4.5V
D1, D2; 100mA
Low
FS = VCC
FS = 0V
High
SD = VCC
FS = 0V, VCC= 4.5V
FS = 0V, VCC= 5.5V
Low
No load, SD = 0V, FS = VCC
High
CONDITIONS
2.52.210FS Input Current500.8FS Input Threshold2.4
4506759001.54.0Switch On-Resistance10Shutdown Input Leakage Current0.8Shutdown Input Threshold2.40.4Shutdown Supply Current (Note 2)
500kHz
Switch Frequency1.15.0Operating Supply Current (Note 1)
UNITSMINTYPMAXPARAMETER

Minimum Start-Up Voltage
solated Transformer Drivefor PCMCIA Applications
OUTPUT RESISTANCE vs. TEMPERATURE
AX845-01
TEMPERATURE (°C)100
VIN = 4.5V
VIN = 5.5V
FIGURE 11c
OUTPUT RESISTANCE vs. TEMPERATURE
AX845-02
TEMPERATURE (°C)100
FIGURE 11b
SHUTDOWN SUPPLY CURRENT
vs. TEMPERATURE
AX845-03
TEMPERATURE (°C)
(µ100
SD = VCC
D1, D2 FREQUENCY vs. TEMPERATURE
MAX845-04
TEMPERATURE (°C)
(k100
VIN = 5.5V
VIN = 4.5V
VIN = 5.0V
VIN = 6.0V
EFFICIENCY vs. LOAD CURRENT
AX845-07
LOAD CURRENT (mA)1008040120
FIGURE 11b
FIGURE 11c
D1, D2 FREQUENCY vs. TEMPERATURE
MAX845-05
TEMPERATURE (°C)100
FS HIGH
VIN = 5.0V
FS LOW
SUPPLY CURRENT vs. TEMPERATURE
MAX845-06
TEMPERATURE (°C)
(m100
VIN = 4.5V
VIN = 5.0V
VIN = 5.5V
VIN = 6.0V
OUTPUT VOLTAGE vs. LOAD CURRENT
AX845-08
LOAD CURRENT (mA)
(V
TRANSFORMERS
USED IN FIGURE 11b
TGM-010P3
TGM-030P3
TGM-020P340
OUTPUT VOLTAGE vs. LOAD CURRENT

AX845-09
LOAD CURRENT (mA)
(V60140120100160
TRANSFORMERS
USED IN FIGURE 11c
TGM-010P3
TGM-030P3
TGM-020P3
__________________________________________Typical Operating Characteristics

(Typical Operating Circuit, VIN= 5V, C1 = 0.1μF, C2 = C3 = 0.33μF, T1 = Halo TGM-010P3, CR1 = CR2 = MBR0520, FS = VCC, = +25°C, unless otherwise noted.)
solated Transformer Driver for PCMCIA Applications
____________________________Typical Operating Characteristics (continued)

(Typical Operating Circuit, VIN= 5V, C1 = 0.1μF, C2 = C3 = 0.33μF, T1 = Halo TGM-010P3, CR1 = CR2 = MBR0520, FS = VCC, = +25°C, unless otherwise noted.)
SWITCHING WAVEFORMS
(TWO CYCLES)

5V/div
400ns/div
SWITCHING WAVEFORM
(BREAK-BEFORE-MAKE)

500mV/div
200ns/div
D2OFFD1OFF
CIRCUIT
OF FIG. 1
D2OND1ON
TIME FROM SHUTDOWN TO POWER-UP

2V/div
5μs/div
OUTPUT
_____________________Pin Description

No Connect. Not internally connected.N.C.5
+5V Supply VoltageVCC6
Ground. Connect both GND1 and GND2 to
ground.GND27
Open Drain of N-Channel Transformer Drive 2D28
Shutdown. Ground for normal operation,
connect to VCCfor shutdown.SD4
Frequency Select (internal pull-up). If FS =
VCCor open, switch frequency = 725kHz; if
FS = 0V, switch frequency = 535kHz.3
PIN

Ground. Connect both GND1 and GND2 to
ground.GND12
Open Drain of N-Channel Transformer Drive 1D11
FUNCTIONNAME

MAX845FS
GND1GND2
VCC7
FREQUENCY
SELECT
ON / OFF
50Ω
50Ω
VINC1
0.1µF
Figure 1. Test Circuit
_______________Detailed Description
The MAX845 is a transformer driver specifically
designed to provide isolated power for PCMCIA and
other height- and/or space-sensitive applications. It
drives a center-tapped transformer primary from a 5V
or 3.3V DC power supply. The secondary can be
wound to provide any isolated DC voltage needed at
power levels up to 750mW.
The 450kHz minimum switching frequency allows the
use of very thin transformers, making the MAX845 ideal
for PCMCIA and other space-limited applications. The
MAX845 is designed to drive a single transformer less
than 0.09 inches (2.3mm) in height, including package.
Further reduction down to 0.050 inches (1.27mm) can
be achieved using a transformer without a package.
The MAX845 consists of an RC oscillator driving a pair
of N-channel power switches. The oscillator runs at
double the output frequency, driving a toggle flip-flop
to ensure 50% duty cycle to each of the switches.
Internal circuitry ensures break-before-make action
between the two switches.
A low-current shutdown mode disables all internal cir-
cuitry, including the oscillator and both power switches.
Drive the shutdown pin (SD) high to shut down the part;
drive SD low for normal operation. The SD pin has no
internal default condition and must not be allowed to
float.
Most MAX845 applications will operate at high frequen-
cies. The frequency-select pin (FS) is pulled high or left
open (FS is internally pulled up to VCC) to operate at a
minimum of 450kHz. Pulling FS low selects the low-fre-
quency state.
Theory of Operation

Figure 2 shows the MAX845 driving both a TGM-010P3
transformer with a center-tapped primary, and a sec-
ondary with a voltage-doubler rectifier topology. All of the
transformers driven by the MAX845 must have a center
tap with VINapplied. Whenever one of the MAX845 out-
puts (D1 or D2) goes low, the other goes to approximate-
ly double the supply voltage. A voltage is induced in the
secondary and the rectifier diodes steer the currents into
the appropriate output capacitor. On alternate half
cycles, each capacitor is charged. The output voltage is
the sum of the voltages from each output capacitor. This
topology yields the simplest and smallest transformer
because the least number of secondary turns is required
for a given voltage.
__________Applications Information

With the MAX845 transformer driver, designers have
the advantages of push/pull converter topology in
space-sensitive applications. The push/pull DC-DC
converter topology allows isolated multiple outputs,
step-up/step-down or inverted outputs, easier filtering
on the input and the output, and lower overall noise.
Isolated Power for PCMCIA Applications

Medical instrumentation, modems, and LAN-interface
cards often require isolated power supplies. One of the
best switching-regulator topologies for this application
is the push/pull forward-converting DC-DC power sup-
ply shown in Figures 3 and 4. Because the transformer
works in the forward mode (rather than the flyback
mode), its core does not store energy and, therefore,
can be small. Input and output capacitors can be small
because of the high-frequency and continuous-current
waveforms.solated Transformer Drive
for PCMCIA Applications

MAX845
GND2GND1
VCC
FREQUENCY
SELECTOUTPUT
5V @ 150mA
OSC
F / F
VIN
ON / OFF
400kHz/
700kHz
ISO
GND
VCCCR1
CR2
Figure 2. Detailed Block Diagram
The MAX845 is a versatile transformer driver, capable
of driving a center-tapped transformer primary from a
5V or 3.3V DC power supply (Figures 3 and 4). The
secondary can be wound to provide any isolated volt-
age needed at power levels up to 750mW with a 5V
supply or up to 500mW with a 3.3V supply. Figure 3
shows a typical 5V to isolated 5V application circuit that
delivers up to 150mA of isolated 5V power.
3.3V Supply

Any of the application circuits shown may be converted
to 3.3V operation by changing the turns ratio of the trans-
former and operating the MAX845 from a boost supply,
as shown in Figure 4. In normal operation, whenever one
of the MAX845 outputs goes low, the other goes to
approximately double the supply voltage. Since the cir-
cuit is symmetrical, the two outputs can be combined
with diodes, lightly filtered, then used to power the
MAX845, and possibly other light loads as well.
The diodes on the primary side may be any fast-switch-
ing small-signal diodes, such as the 1N914, 1N4148, or
CMPD2838. The value of the primary filter capacitor is
not critical and can be very small, since it only needs to
supply current to the MAX845 during the break-before-
make interval.
The transformer could be any of the same ones used for
5V operation, but for optimum performance it should
have fewer primary turns, as the ET product required is
now only 3.3V-μs. For a given power level, the currents
will be higher at 3.3V, so transformer winding resistance
will be more critical and efficiencies will be lower. The
MAX845 output current must still be limited to 200mA
(see Absolute Maximum Ratings), so the available out-
put power will be less than with a 5V power source.
Low-Noise Power Supply

The MAX845 topology is inherently low noise, in that
either one or the other of the two power devices is on at
any given time. By alternating between two identical
states with one side on and the other off, the input cur-
rent is nearly constant and secondary output power is
available at all times. There is an intentional break-
before-make action to prevent any possibility of both
power switches conducting at the same time. During
this 100ns non-overlap interval, the input current goes
to zero. This adds a small high-frequency component
to the input current waveform. This ripple current can
easily be absorbed by a small input bypass capacitor
(0.33μF) from VCCto ground. Figure 5 shows a low-
noise bias supply using the MAX845 transformer driver.
When using the two-diode push-pull (Figure 11a)
rectifier or the four-diode bridge (Figure 11b), the out-
put voltage tends to be more constant than in most
alternative topologies. As described above, the circuit
alternates between two identical states that both pro-
vide power to the load. The only part of the cycle that
produces output ripple is the 100ns non-overlap inter-
val, which can easily be filtered by a small ceramic
output capacitor (0.33μF).solated Transformer Driver
for PCMCIA Applications

0.33µF
0.1µF
5V @ 150mA
ISO OUTPUT
VIN
MAX845FS
GND1GND2
VCC17
FREQUENCY
SELECT4
ISO
GND
MBR0520
1CT:1.3CT
MBR0520
ON / OFF
Figure 3. 5V to Isolated 5V Application Circuit
MAX845
GND1GND2
VCC17
1N4148
1N4148
3.3V
SUPPLY
SEE FIGURE 11
FOR RECTIFIER
CONFIGURATIONS
0.01µF
Figure 4. 3.3V Input to Isolated Output Application Circuit
Isolated Data Conversion
Almost any serial-interface device is a candidate for
operation across an isolation barrier; Figure 6 illustrates
one example. The MAX176 analog-to-digital converter
(ADC) operates from +5V and -12V supplies, provided
by the multiple-tapped secondary and linear regulators.
This circuit easily supplies several hundred milliwatts of
additional isolated power for signal conditioning, multi-
plexing, or sensors. A +12V supply can be generated
by adding two more diodes from the ends of the sec-
ondary, and a -5V supply can be generated by con-
necting additional diodes to the 1⁄4and 3⁄4tap points on
the secondary. The MAX845 supplies sufficient power
for almost any Maxim ADC.
Telephone-Subscriber-Line Power Supply

The standard telephone system is placed in the “off
hook” state by placing a load on the line to signal the
central office that service is requested. Normally, most of
this power is wasted in a load resistor, but some systems
can benefit from utilizing this free power. Figure 7 shows
one way to transform the wasted telephone power to an
isolated, regulated 5V at currents up to 50mA.
Because the telephone line is a high-impedance
source, there can be a start-up problem with any DC-
to-DC converter; when the line voltage is low during
start-up, the frequency can be too low for the trans-
former, causing it to saturate. This excess saturation
current can keep the voltage from climbing to normal
operating levels. Thus the purpose of Q1, Q2, and the
associated resistors is to ensure that the MAX845
remains in the shutdown mode until the voltage is high
enough to allow proper operation.
Isolated 4mA to 20mA Analog Interface

The 4mA to 20mA current loop is widely used in the
process-control industry for transducer and actuator
control signals. These signals are commonly referred to
a distant ground that may be at a considerably higher
voltage with respect to the local ground. The circuit in
Figure 8 generates an isolated 4mA to 20mA current
from a 5V supply.
Isolated RS-485 Data Interface

The MAX845 power-supply transformer driver also pro-
vides isolated power for RS-485 data-interface applica-
tions. The application circuit of Figure 9 combines the
MAX845 with a low-dropout linear regulator, a trans-
former, several high-speed optocouplers, and a Maxim
RS-485 interface device.
Isolated RS-232 Data Interface

The MAX845 is ideal for isolated RS-232 data-interface
applications requiring more than four transceivers. Its
750mW output power capability enables it to drive 10
transceivers simultaneously. Figure 10 shows the typi-
cal application circuit for a complete 120kbps isolated
RS-232 data interface. This figure also shows how the
Sharp PC417 optocouplers can be replaced by the
lower-cost Quality Technologies 4N25 devices to
achieve data transfer rates up to 19.2kbps. solated Transformer Drive
for PCMCIA Applications

0.33µF
MAX845
GND1GND2N.C.27
VCC6
MBR0520L*
*1N914 POSSIBLE FOR LOWER CURRENTS
0.33µFOUT
GND
78L05
-5V
100mA
HALO
TGM-030P3
N.C.
Figure 5. Low-Noise Supply
solated Transformer Driver for PCMCIA Applications
10µF
6N136
MAX845
GND1GND2
VCC7
6N136
6N136
10µF
79L12
78L05
CONVST
VDDVSS
CLOCK
DATA
AIN
VREF
GND
MAX176
470Ω
0.1µF10µF
0.1µF
10µF
0.1µF
10µF
200Ω
200Ω
8.2k
74HC04
SER
SCK
RCK
SCLR
74HC5958
INPUT
0.1µF
D11(MSB)
D10
INPUT
SER
SCK
RCK
SCLR
74HC5958
INPUT
0.1µF
D0 (LSB)
INPUT
74HC04
ON/OFF
START
INPUT CLOCK
1CT : 1.5CT : 3CT
4 x 1N5817
VIN
5V INPUT
SIGNAL
GROUND
ANALOG
INPUT
ISO
ISO
-12V
QH′
ISOLATION
BARRIER
Figure 6. Typical Isolated Data-Conversion Application
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