PJ3842B / PJ3843B
High Performance Current Mode Controller
7-14
2004/11.Rev B
FIGURE 18-TIMING DIAGRAM
Two undervoltage lockout comparators have been
incorporated to guarantee that the IC is fully functional before
the output stage is enabled. The positive power supply
terminal (VCC) and the reference output (Vref) are each
monitored by separate comparators.Each has built-in
hysteresis to prevent erratic output behavior as their respective
thresholds are crossed. The large hysteresis and low start-up
current of the PJ3842B makes it ideally suited in off-line
converter applications where efficient bootstrap start-up
technique (Figure 33). 36 V zener is connected as a shunt
regulator from VCC to ground.Its purpose is to protect the IC
from excessive voltage that can occur during system start-up.
The minimum operating voltage for the PJ3842B is 11V.
Output
These devices contain a single totem pole output stage
that was specifically designed for direct drive of power
MOSFET’s. It is capable of up to ±1.0A peak drive current
and has a typical rise and fall time of 50 ns with a 1.0nF load.
Additional internal circuitry has been added to keep the
Output in a sinking mode whenever an undervoltage lockout
is active.This characteristic eliminates the need for an external
pull-down resistor.
The SOP-8 surface mount package provides separate pins
for
Vc(output
supply)
and
Power
Ground.Proper
implementation will significantly reduce the level of
switching transient noise imposed on the control circuitry.
This becomes particularly useful when reducing the Ipk(max)
clamp level.The separate Vc supply input allows the designer
added fiexlbility in tailoring the drive voltage independent of
Vcc.A zener clamp is typically connected to this input when
driving power MOSFETs in systems where Vcc is greater
than 20V. Figure 25 shows proper power and control ground
connections in a current sensing power MOSFET application.
Reference
The 5.0V bandgap reference is trimmed to±2.0% on the
PJ3842B.Its promary purpose to supply charging current to
the oscillator timing capacitor.The reference has short circuit
protection and is capable of providing in excess of 20mA for
powering additional control system circuitry.
UNDERVOLTAGE LOCKOUT
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