19-0372; Rev 4; 10/97
KIT
ATION
EVALU
E
BL
AVAILA
Isolated Transformer Driver
for PCMCIA Applications
____________________________Features
o
Transformer Driver for Ultra-Thin 5V-µs Transformers
o
Isolated DC-to-DC Power Supply for PCMCIA
Applications
o
450kHz Minimum Switching Frequency
o
Ultra-Low Input Supply Current Ripple
o
Single +5V or +3.3V Supply
o
5µW Low-Power Shutdown Mode
o
8-Pin SO and µMAX Packages
o
Low Output Ripple Permits Miniature Output
Capacitors
_______________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.
MAX845
________________________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
_______________Ordering Information
PART
MAX845C/D
MAX845ESA
MAX845EUA
TEMP. RANGE
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
Dice*
8 SO
8 µMAX
*Contact factory for dice specifications.
__________Typical Operating Circuit
ON / OFF
4
SD
6
V
CC
D1
1
V
IN
5V
C1
CR1
OUTPUT
5V @ 150mA
___________________Pin Configuration
TOP VIEW
D1
C2
1
2
8
7
D2
GND2
V
CC
N.C.
MAX845
3
FREQUENCY
SELECT
FS
GND1
2
D2
GND2
7
CR2
8
T1
C3
GND1
FS
3
SD
4
MAX845
6
5
SO/µMAX
________________________________________________________________
Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 408-737-7600 ext. 3468.
Isolated Transformer Driver
for PCMCIA Applications
MAX845
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (V
CC
) ...............................................-0.3V to +7V
Control Input Voltage (SD, FS) ...................-0.3V to (V
CC
+ 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 (T
A
= +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
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.
ELECTRICAL CHARACTERISTICS
(V
CC
= 5V ±10%, T
A
= T
MIN
to T
MAX
, unless otherwise noted. Typical values are at T
A
= +25°C.)
PARAMETER
Switch On-Resistance
D1, D2; 100mA
FS = V
CC
= 4.5V
Switch Frequency
FS = V
CC
= 5.5V
FS = 0V, V
CC
= 4.5V
FS = 0V, V
CC
= 5.5V
Operating Supply Current (Note 1)
Shutdown Supply Current (Note 2)
Shutdown Input Threshold
Shutdown Input Leakage Current
FS Input Threshold
FS Input Current
Minimum Start-Up Voltage
High
Low
FS = 0V
FS = V
CC
2.5
10
2.2
2.4
0.8
50
No load, SD = 0V, FS = V
CC
SD = V
CC
High
Low
10
2.4
0.8
450
550
CONDITIONS
MIN
TYP
1.5
675
860
500
575
1.1
0.4
5.0
mA
µA
V
pA
V
µA
V
MAX
4.0
900
1100
kHz
UNITS
Ω
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.
2
_______________________________________________________________________________________
Isolated Transformer Driver
for PCMCIA Applications
__________________________________________Typical Operating Characteristics
(Typical
Operating Circuit,
V
IN
= 5V, C1 = 0.1µF, C2 = C3 = 0.33µF, T1 = Halo TGM-010P3, CR1 = CR2 = MBR0520, FS = V
CC
,
T
A
= +25°C, unless otherwise noted.)
SHUTDOWN SUPPLY CURRENT
vs. TEMPERATURE
SD = V
CC
1.4
SHUTDOWN CURRENT (µA)
1.2
1.0
0.8
0.6
0.4
0.2
-40
-20
0
20
40
60
80
100
-40
-20
0
20
40
60
80
100
TEMPERATURE (°C)
TEMPERATURE (°C)
MAX845-03
MAX845
OUTPUT RESISTANCE vs. TEMPERATURE
MAX845-01
OUTPUT RESISTANCE vs. TEMPERATURE
7.0
OUTPUT RESISTANCE (Ω)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
FIGURE 11b
MAX845-02
40
FIGURE 11c
OUTPUT RESISTANCE (Ω)
35
30
25
20
15
10
-40
-20
0
20
40
60
80
V
IN
= 5.5V
V
IN
= 4.5V
7.5
1.6
2.5
100
TEMPERATURE (°C)
D1, D2 FREQUENCY vs. TEMPERATURE
MAX845-04
D1, D2 FREQUENCY vs. TEMPERATURE
V
IN
= 5.0V
800
FREQUENCY (kHz)
750
700
650
600
550
500
-40
FS LOW
-20
0
20
40
60
80
100
FS HIGH
MAX845-05
SUPPLY CURRENT vs. TEMPERATURE
1.6
SUPPLY CURRENT (mA)
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
-40
-20
0
20
40
60
80
100
V
IN
= 4.5V
V
IN
= 5.0V
V
IN
= 5.5V
V
IN
= 6.0V
MAX845-06
1000
950
900
FREQUENCY (kHz)
850
800
750
700
650
600
-40
-20
60
V
IN
= 4.5V
V
IN
= 6.0V
V
IN
= 5.5V
V
IN
= 5.0V
850
1.7
0
20
40
80
100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
EFFICIENCY vs. LOAD CURRENT
MAX845-07
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX845-08
OUTPUT VOLTAGE vs. LOAD CURRENT
14
13
OUTPUT VOLTAGE (V)
TRANSFORMERS
USED IN FIGURE 11c
TGM-030P3
MAX845-09
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0
20
40
60
80
FIGURE 11c
FIGURE 11b
7.5
7.0
6.5
OUTPUT VOLTAGE (V)
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
TGM-020P3
TGM-010P3
TGM-030P3
TRANSFORMERS
USED IN FIGURE 11b
15
12
11
10
9
8
7
6
5
TGM-010P3
TGM-020P3
100 120 140 160
0
20
40
60
80
100 120 140 160
0
20
40
60
80
100 120 140 160
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
_______________________________________________________________________________________
3
Isolated Transformer Driver
for PCMCIA Applications
MAX845
____________________________Typical Operating Characteristics (continued)
(Typical
Operating Circuit,
V
IN
= 5V, C1 = 0.1µF, C2 = C3 = 0.33µF, T1 = Halo TGM-010P3, CR1 = CR2 = MBR0520, FS = V
CC
,
T
A
= +25°C, unless otherwise noted.)
SWITCHING WAVEFORMS
(TWO CYCLES)
SWITCHING WAVEFORM
(BREAK-BEFORE-MAKE)
D1OFF
D2OFF
D1
5V/div
500mV/div
D2
CIRCUIT
OF FIG. 1
D2ON
400ns/div
D1ON
200ns/div
TIME FROM SHUTDOWN TO POWER-UP
SD
2V/div
OUTPUT
5µs/div
_____________________Pin Description
V
IN
PIN
1
2
3
4
5
6
7
8
4
NAME
D1
GND1
FS
SD
N.C.
V
CC
GND2
D2
FUNCTION
Open Drain of N-Channel Transformer Drive 1
Ground. Connect both GND1 and GND2 to
ground.
Frequency Select (internal pull-up). If FS =
V
CC
or open, switch frequency = 725kHz; if
FS = 0V, switch frequency = 535kHz.
Shutdown. Ground for normal operation,
connect to V
CC
for shutdown.
No Connect. Not internally connected.
+5V Supply Voltage
Ground. Connect both GND1 and GND2 to
ground.
Open Drain of N-Channel Transformer Drive 2
5V
C1
0.1µF
4
ON / OFF
6
V
CC
SD
D1
R1
50Ω
1
R2
50Ω
D2
8
MAX845
3
FREQUENCY
SELECT
FS
GND1
2
GND2
7
Figure 1. Test Circuit
_______________________________________________________________________________________
Isolated Transformer Driver
for PCMCIA Applications
MAX845
V
IN
C1
V
CC
F/F
V
CC
Q
T
OSC
400kHz/
700kHz
Q
N
SD
ON / OFF
GND2
GND1
ISO
GND
D2
CR2
C3
N
D1
5V
CR1
OUTPUT
5V @ 150mA
MAX845
FS
FREQUENCY
SELECT
C2
Figure 2. Detailed Block Diagram
_______________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 V
CC
) 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 V
IN
applied. 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.
5
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