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IMP38C4X Datasheet(PDF) 6 Page - IMP, Inc

Part No. IMP38C4X
Description  BiCMOS Current-Mode PWM Controllers
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Maker  IMP [IMP, Inc]
Homepage  http://www.ds-imp.com.cn/index.asp
Logo IMP - IMP, Inc

IMP38C4X Datasheet(HTML) 6 Page - IMP, Inc

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IMP38C/HC/42/3/4/5
IMP38C/HC/42/3/4/5
6
408-432-9100/www.impweb.com
© 2000 IMP, Inc. !
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
current-sense amplifier. VREF and VCC stability will be aided by
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 VDD drops to zero volts, the OUT pin will not rise
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
VDD (V)
2.0
1.5
1.0
0.5
0
VOUT While Sinking 20mA
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38C/HC_05b.eps
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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-
ciency. In addition, the VCC hold-up capacitance (used during
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 VCC = 14V, RT = 10k and CT = 3.3nF, nominal switching
frequency is 50kHz.
Figure 1. Oscillator Component Selection
Application Information
frequency Hz
1.73
RT
CT F
[] = []× []
IMP38C4x
IMP38HC4x
VREF
RT/CT
GND
38C/HC_07.eps
RT
CT


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