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UC3843AD1R2G Datasheet(PDF) 10 Page - ON Semiconductor

Part # UC3843AD1R2G
Description  High Performance Current Mode Controllers
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

UC3843AD1R2G Datasheet(HTML) 10 Page - ON Semiconductor

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UC3842A, UC3843A, UC2842A, UC2843A
http://onsemi.com
10
PIN FUNCTION DESCRIPTION
Pin
Function
Description
8−Pin
14−Pin
1
1
Compensation
This pin is Error Amplifier output and is made available for loop compensation.
2
3
Voltage
Feedback
This is the inverting input of the Error Amplifier. It is normally connected to the switching pow-
er supply output through a resistor divider.
3
5
Current Sense
A voltage proportional to inductor current is connected to this input. The PWM uses this infor-
mation to terminate the output switch conduction.
4
7
RT/CT
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.
5
GND
This pin is the combined control circuitry and power ground (8−pin package only).
6
10
Output
This output directly drives the gate of a power MOSFET. Peak currents up to 1.0 A are
sourced and sunk by this pin.
7
12
VCC
This pin is the positive supply of the control IC.
8
14
Vref
This is the reference output. It provides charging current for capacitor CT through
resistor RT.
8
Power Ground
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.
11
VC
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.
9
GND
This pin is the control circuitry ground return (14−pin package only) and is connected back to
the power source ground.
2,4,6,13
NC
No connection (14−pin package only). These pins are not internally connected.
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 34). 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 SOIC−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 than 20 V. Figure 26 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.


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