IMP38C/HC/42/3/4/5

IMP38C/HC/42/3/4/5

6

408-432-9100/www.impweb.com

S

Transient Protection

Good high-frequency layout practices should be followed. Avoid

long printed-circuit traces and component leads. Locate oscillator

and compensation components near the IC. Avoid capacitive and

inductive coupling of the switching waveform into the

high-impedance inputs of the error amplifier, oscillator, and

using high-frequency decoupling capacitors.

Undervoltage Lockout

Several different thresholds are available.

Under-voltage lockout (UVL) performance has been improved.

When the power supply voltage is below the startup voltage,

internal circuitry puts the output into a low impedance state and

sets the output to zero. The output will sink up to 20mA in this

controlled state and remain below 1.3V, well below the turn-on

threshold voltage of the external MOSFET.

With the IMP38C/HC4x output stage design, leakage current

from the external power MOSFETs will not force the PWM output

to rise and turn the external MOSFET on.

above approximately 1.3V because the UVLO circuit is powered

through the OUT pin.

The IMP output stage performance in UVL mode, while sinking

20mA, is shown in Figure 2.

Figure 2. Output Stage Performance in UVL Mode

0

2.0

1.5

1.0

0.5

0

12345

7

69

8

38C/HC_05b.eps

IMP38C/HC4x

s

d

l

o

h

s

e

r

h

T

t

u

o

k

c

o

L

e

g

a

t

l

o

v

r

e

d

n

U

V

4

.

8

t

a

p

u

-t

r

a

t

S

V

6

.

7

=

m

u

m

i

n

i

M

g

n

i

t

a

r

e

p

O

V

5

.

4

1

t

a

p

u

-t

r

a

t

S

V

0

.

9

=

m

u

m

i

n

i

M

g

n

i

t

a

r

e

p

O

3

4

C

H

/

3

4

C

8

3

P

M

I2

4

C

H

/

2

4

C

8

3

P

M

I

5

4

C

H

/

5

4

C

8

3

P

M

I4

4

C

H

/

4

4

C

8

3

P

M

I

s

p

e

.

4

0

t

_

C

H

/

C

8

3

The IMP38C and IMP38HC devices are compatible with generic

384x PWM devices. The following discussion highlights the dif-

ferences and advantages of the IMP ‘C’ and ‘HC’ designs.

Start-up Current

IMP’s BiCMOS process allows for substantial reduction in the

start-up current. Typical start-up current is 95

µA, with a maxi-

mum limit of 120

µA. Low start-up current allows high resistance,

lower-wattage, start-up resistors to supply controller start-up

power.

Operating Current

Operating current has been reduced to 1.5mA maximum.

Compared to the 11mA needed for a typical bipolar controller and

6mA for competitive BiCMOS controllers, IMP's low operating

current allows the controller to run cooler and with better effi-

start-up) can be reduced.

Output Driver

The IMP38HC4x/38C4x CMOS output stage drives external

power MOSFETs to the full supply voltage. Low ON-resistance

and high peak current drive combine to give greater than 1000pF

gate capacitance drive capability. Rise and fall time requirements

may dictate the appropriate value of output capacitance. Within

the restrictions of output capacity and controller power dissipa-

tion, switching frequencies can exceed 1MHz.

The CMOS output stage “break-before-make” action is guaran-

teed by design and insures that no cross-conduction current will

flow. This minimizes heat dissipation, increases efficiency and

enhances reliability.

Oscillator Operation

Two external components, RT and CT, set the switching frequency.

frequency is 50kHz.

Figure 1. Oscillator Component Selection

Application Information

frequency Hz

1.73

RT

CT F

Ω

IMP38C4x

IMP38HC4x

RT/CT

GND

38C/HC_07.eps

RT

CT