14

LT1336

most power supplies cannot sink current, this current can

raise the voltage of the high voltage rail. This can be

avoided by placing a discharge resistor between HV sup-

ply and ground to divert the return current to ground as

shown in Figure 5. For a high voltage rail of 40V, a 26k

resistor or smaller should be used, since the top driver will

return about 1.5mA.

For applications where using a discharge resistor is unde-

sirable, use the flyback regulator topology instead of the

boost regulator topology (see Deriving the Floating Supply

with the Flyback Topology).

To drive a DC motor in both directions, two LT1336s can

be used to drive an H-bridge output stage. In this configu-

ration the motor can be made to run clockwise, counter-

clockwise, stop rapidly (“plugging” braking) or free run

(coast) to a stop. A very rapid stop may be achieved by

reversing the current, though this requires more careful

design to stop the motor dead. In practice a closed-loop

control system with tachometric feedback is usually

necessary.

The motor speed in these examples can be controlled by

switching the drivers with pulse width modulated square

waves. This approach is particularly suitable for micro-

computers/DSP control loops.

Figure 6. 90% Efficiency, 40V to 5V, 10A, Low Dropout Voltage Mode Switching Regulator

SWITCH

BOOST

TGATEDR

TGATEFB

TSOURCE

BGATEDR

BGATEFB

PGND

LT1336

D1

1N4148

D2

1N4148

1

µF

1336 F05

HV = 40V MAX

+

+

2

Ω

1/4W

24k

*SUMIDA

RCR-664D-221KC

200

µH*

10

µF

+

–

+

Figure 5. Driving a Supply Referenced Motor

+

+

+

+

+

+

2.2nF

27k

0.1

µF

1k

1k

5k

2k

5V

5400

µF

LOW

ESR

1336 F06

1

µF

10

µF

1

µF

1

µF

0.022

µF

C1

0.1

µF

360

Ω

510

Ω

4.7k

0.33

µF

330k

12V

1N4148

1N4148

IRFZ44

IRFZ44

MBR340

* SUMIDA RCR-664D-221KC

** MAGNETICS CORE #55585-A2 30 TURNS 14GA MAGNET WIRE

IRFZ44

40V

2200

µF EA

LOW ESR

†

0.007

Ω

L2**

70

µH

f = 25kHz

16

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

L1*

200

µH

2

Ω, 1/4W

SWITCH

SWGND

BOOST

TGATEDR

TGATEFB

TSOURCE

BGATEDR

LT1336

INTOP

INBOTTOM

UVOUT

SGND

PGND

BGATEFB

SHUTDOWN

+

2N2222

1N4148

0.1

µF

4.7k

1

2

3

4

5

6

7

8

9

18

17

16

15

14

13

12

11

10

LT3526

1N4148