UC3842A, 43A UC2842A, 43A
PIN FUNCTION DESCRIPTION
Pin
8–Pin
1
2
3
4
5
6
7
8
–
–
14–Pin
1
3
5
7
–
10
12
14
8
11
Function
F
i
Compensation
Voltage
Feedback
Current Sense
RT/CT
Gnd
Output
VCC
Vref
Power Ground
VC
Description
D
i i
This pin is Error Amplifier output and is made available for loop compensation.
This is the inverting input of the Error Amplifier. It is normally connected to the switching power
supply output through a resistor divider.
A voltage proportional to inductor current is connected to this input. The PWM uses this
information to terminate the output switch conduction.
The Oscillator frequency and maximum Output duty cycle are programmed by connecting
resistor RT to Vref and capacitor CT to ground. Operation to 500 kHz is possible.
This pin is the combined control circuitry and power ground (8–pin package only).
This output directly drives the gate of a power MOSFET. Peak currents up to 1.0 A are sourced
and sunk by this pin.
This pin is the positive supply of the control IC.
This is the reference output. It provides charging current for capacitor CT through
resistor RT.
This pin is a separate power ground return (14–pin package only) that is connected back to the
power source. It is used to reduce the effects of switching transient noise on the control circuitry.
The Output high state (VOH) is set by the voltage applied to this pin (14–pin package only). With
a separate power source connection, it can reduce the effects of switching transient noise on the
control circuitry.
This pin is the control circuitry ground return (14–pin package only) and is connected back to the
power source ground.
No connection (14–pin package only). These pins are not internally connected.
–
–
9
2,4,6,13
Gnd
NC
Undervoltage Lockout
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 VCC comparator
upper and lower thresholds are 16 V/10 V for the UCX842A,
and 8.4 V/7.6 V for the UCX843A. The Vref comparator upper
and lower thresholds are 3.6V/3.4 V. The large hysteresis
and low startup current of the UCX842A makes it ideally
suited in off–line converter applications where efficient
bootstrap startup techniques are required (Figure 33). The
UCX843A is intended for lower voltage dc to dc converter
applications. A 36 V zener is connected as a shunt regulator
form VCC to ground. Its purpose is to protect the IC from
excessive voltage that can occur during system startup. The
minimum operating voltage for the UCX842A is 11 V and
8.2 V for the UCX843A.
Output
These devices contain a single totem pole output stage
that was specifically designed for direct drive of power
MOSFETs. It is capable of up to
±1.0
A peak drive current and
has a typical rise and fall time of 50 ns with a 1.0 nF 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 SO–14 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 flexibility 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 that
20 V. Figure 25 shows proper power and control ground
connections in a current sensing power MOSFET
application.
Reference
The 5.0 V bandgap reference is trimmed to
±1.0%
tolerance at TJ = 25°C on the UC284XA, and
±
2.0% on the
UC384XA. Its primary purpose is to supply charging current
to the oscillator timing capacitor. The reference has short
circuit protection and is capable of providing in excess of
20 mA for powering additional control system circuitry.
MOTOROLA ANALOG IC DEVICE DATA
9
ONSEMI [ ON SEMICONDUCTOR ]
