IMP38C4X IMP Inc, IMP38C4X Datasheet - Page 6

• Manufacturer Part Number: IMP38C4X
• Description: BiCMOS Current-Mode PWM Controllers
• Manufacturer: IMP Inc

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 95µA, with a maxi-
mum limit of 120µA. 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-
ciency. In addition, the V
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.
With V
frequency is 50kHz.
6
frequency Hz
Application Information
I M P 3 8 C / H C / 4 2 / 3 / 4 / 5
I M P 3 8 C / H C / 4 2 / 3 / 4 / 5
CC
[ ]
Figure 1. Oscillator Component Selection
= 14V, RT = 10k and CT = 3.3nF, nominal switching
=
RT
[ ]
Ω
1.73
×
CT F
[ ]
CC
IMP38HC4x
IMP38C4x
hold-up capacitance (used during
RT/CT
GND
V
REF
38C/HC_07.eps
RT
CT
408-432-9100/www.impweb.com
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
current-sense amplifier. V
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.
Even when V
above approximately 1.3V because the UVLO circuit is powered
through the OUT pin.
20mA, is shown in Figure 2.
The IMP output stage performance in UVL mode, while sinking
Figure 2. Output Stage Performance in UVL Mode
I
I
M
M
P
P
2.0
1.5
1.0
0.5
3
3
0
8
8
0
DD
C
C
4
4
3
5
drops to zero volts, the OUT pin will not rise
1
H /
H /
C
C
V
4
4
2
OUT
3
5
REF
While Sinking 20mA
3
and V
V
4
DD
(V)
CC
5
stability will be aided by
I
I
M
M
IMP38C/HC4x
6
P
P
3
3
8
8
C
C
7
4
4
2
4
38C/HC_05b.eps
H /
H /
8
© 2000 IMP, Inc.
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