Maxim MAX845EUA, MAX845C-D, MAX845ESA Datasheet

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_______________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 trans­former primary from a 5V or 3.3V DC power supply. The
secondary can be wound to provide any isolated posi­tive or negative voltage at powers up to 750mW.

The MAX845 consists of an oscillator followed by a tog­gle 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 switch­es. 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 evalua­tion kit (MAX845EVKIT-MM) is available to evaluate low­profile 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 Frequency
♦ Ultra-Low Input Supply Current Ripple
♦ Single +5V or +3.3V Supply
♦ 5µW Low-Power Shutdown Mode
♦ 8-Pin SO and µMAX Packages
♦ Low Output Ripple Permits Miniature Output

Capacitors

MAX845

Isolated Transformer Driver

for PCMCIA Applications

________________________________________________________________

Maxim Integrated Products

1

1
2
3
4

8
7
6
5

D2
GND2
V

CC

N.C.

SD

FS

GND1

D1

SO/µMAX

TOP VIEW

MAX845

___________________Pin Configuration

MAX845

D1

D2FS

GND1 GND2

V

CC

1

8

46

27

3

V

IN

SD

FREQUENCY

SELECT

C2

C1

C3

5V @ 150mA

OUTPUT

5V

ON / OFF

T1

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. RANGE PIN-PACKAGE

Dice*

8 µMAX

EVALUATION KIT

AVAILABLE

*Contact factory for dice specifications.

_______________Ordering Information

MAX845ESA -40°C to +85°C 8 SO

MAX845

Isolated Transformer Driver
for PCMCIA Applications

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VCC= 5V ±10%, TA= T

MIN

to T

MAX

, 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 (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

FS = V

CC

= 5.5V

FS = VCC= 4.5V

D1, D2; 100mA

Low

FS = V

CC

FS = 0V

High

SD = V

CC

FS = 0V, VCC= 4.5V
FS = 0V, VCC= 5.5V

Low

No load, SD = 0V, FS = V

CC

High

CONDITIONS

V2.5 2.2

µA

10

FS Input Current

50

V

0.8

FS Input Threshold

2.4

550 860 1100

450 675 900

Ω1.5 4.0Switch On-Resistance

pA10Shutdown Input Leakage Current

V

0.8

Shutdown Input Threshold

2.4

µA0.4Shutdown Supply Current (Note 2)

500

kHz

575

Switch Frequency

mA1.1 5.0Operating Supply Current (Note 1)

UNITSMIN TYP MAXPARAMETER

Minimum Start-Up Voltage

MAX845

Isolated Transformer Driver

for PCMCIA Applications

_______________________________________________________________________________________

3

40

-20 60

OUTPUT RESISTANCE vs. TEMPERATURE

30

MAX845-01

TEMPERATURE (°C)

OUTPUT RESISTANCE (Ω)

20 100

20

15

10

35

25

-40 0 8040

VIN = 4.5V

VIN = 5.5V

FIGURE 11c

7.5

2.5

-20 60

OUTPUT RESISTANCE vs. TEMPERATURE

3.5

6.0

MAX845-02

TEMPERATURE (°C)

OUTPUT RESISTANCE (Ω)

20 100

5.0

4.5

3.0

4.0

7.0

6.5

5.5

-40 0 8040

FIGURE 11b

1.6

1.4

1.2

0.2

-20 60

SHUTDOWN SUPPLY CURRENT

vs. TEMPERATURE

MAX845-03

TEMPERATURE (°C)

SHUTDOWN CURRENT (µA)

20 100

0.6

1.0

-40 0 8040

0.8

0.4

SD = V

CC

1000

950

600

-20 60

D1, D2 FREQUENCY vs. TEMPERATURE

MAX845-04

TEMPERATURE (°C)

FREQUENCY (kHz)

20 100

750

900
850

-40 0 8040

800

700
650

VIN = 5.5V

VIN = 4.5V

VIN = 5.0V

VIN = 6.0V

90

100

0

60

140 160

EFFICIENCY vs. LOAD CURRENT

20

70

MAX845-07

LOAD CURRENT (mA)

EFFICIENCY (%)

20 100

50
40

10

30

80

60

0

8040

120

FIGURE 11b

FIGURE 11c

850

800

500

550

-20 60

D1, D2 FREQUENCY vs. TEMPERATURE

MAX845-05

TEMPERATURE (°C)

FREQUENCY (kHz)

20 100

650

750

-40

0 8040

700

600

FS HIGH

VIN = 5.0V

FS LOW

1.5

1.4

0.8

-20 60

SUPPLY CURRENT vs. TEMPERATURE

MAX845-06

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

20 100

1.0

-40 0 8040

0.9

1.1

1.3

1.2

1.7

1.6

VIN = 4.5V

VIN = 5.0V

VIN = 5.5V

VIN = 6.0V

7.5

2.5
0 40

OUTPUT VOLTAGE vs. LOAD CURRENT

3.5

6.5

MAX845-08

LOAD CURRENT (mA)

OUTPUT VOLTAGE (V)

80

5.5

4.5

3.0

4.0

7.0

6.0

5.0

20 60 140120100 160

TRANSFORMERS

USED IN FIGURE 11b

TGM-010P3

TGM-030P3

TGM-020P3

15

5

0 40

OUTPUT VOLTAGE vs. LOAD CURRENT

7

13

MAX845-09

LOAD CURRENT (mA)

OUTPUT VOLTAGE (V)

80

11

9

6

8

14

12

10

20 60 140120100 160

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,

T

A

= +25°C, unless otherwise noted.)

MAX845

Isolated Transformer Driver
for PCMCIA Applications

4 _______________________________________________________________________________________

____________________________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,

TA= +25°C, unless otherwise noted.)

SWITCHING WAVEFORMS

(TWO CYCLES)

5V/div

400ns/div

D1

D2

SWITCHING WAVEFORM
(BREAK-BEFORE-MAKE)

500mV/div

200ns/div

D2OFFD1OFF

CIRCUIT

OF FIG. 1

D2ON D1ON

TIME FROM SHUTDOWN TO POWER-UP

2V/div

5µs/div

SD

OUTPUT

_____________________Pin Description

No Connect. Not internally connected.N.C.5
+5V Supply VoltageV

CC

6

Ground. Connect both GND1 and GND2 to
ground.

GND27

Open Drain of N-Channel Transformer Drive 2D28

Shutdown. Ground for normal operation,
connect to V

CC

for shutdown.

SD4

Frequency Select (internal pull-up). If FS =
VCCor open, switch frequency = 725kHz; if
FS = 0V, switch frequency = 535kHz.

FS3

PIN

Ground. Connect both GND1 and GND2 to
ground.

GND12

Open Drain of N-Channel Transformer Drive 1D11

FUNCTIONNAME

Figure 1. Test Circuit

V

IN

5V

ON / OFF

FREQUENCY

SELECT

C1

0.1µF

4

SD

3

6

V

CC

MAX845

GND1 GND2

2 7

R1

50Ω

1

D1

R2

50Ω

8

D2FS

_______________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 cir­cuitry, 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 frequen­cies. 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-fre­quency state.

Theory of Operation

Figure 2 shows the MAX845 driving both a TGM-010P3
transformer with a center-tapped primary, and a sec­ondary 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 out­puts (D1 or D2) goes low, the other goes to approximate­ly 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 sup­ply 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.

MAX845

Isolated Transformer Driver

for PCMCIA Applications

_______________________________________________________________________________________ 5

MAX845

D1

D2

FS

GND2 GND1

V

CC

FREQUENCY

SELECT

C2

C3

C1

OUTPUT

5V @ 150mA

5V

N

N

Q

Q

OSC

F / F

V

IN

SD

ON / OFF

400kHz/

700kHz

T

ISO
GND

V

CC

CR1

CR2

Figure 2. Detailed Block Diagram