UC3844, UC3845, UC2844, UC2845
V
CC
, OUTPUT VOLTAGE
25
20 V/DIV
I
CC
, SUPPLY CURRENT (mA)
V
CC
= 30 V
C
L
= 15 pF
T
A
= 255C
20
15
10
UCX845
5
0
100 ns/DIV
UCX844
R
T
= 10 k
C
T
= 3.3 nF
V
FB
= 0 V
I
Sense
= 0 V
T
A
= 255C
40
I
CC
, SUPPLY CURRENT
100 mA/DIV
0
10
20
30
V
CC
, SUPPLY VOLTAGE (V)
Figure 14. Output Cross Conduction
Figure 15. Supply Current versus
Supply Voltage
PIN FUNCTION DESCRIPTION
Pin
8−Pin
1
2
3
4
5
6
7
8
−
14−Pin
1
3
5
7
−
10
12
14
8
Function
Compensation
Voltage
Feedback
Current Sense
R
T
/C
T
GND
Output
V
CC
V
ref
Power Ground
Description
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 R
T
to V
ref
and capacitor C
T
to ground. Operation to 1.0 MHz is possible.
This pin is 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. The output switches at one−half the oscillator frequency.
This pin is the positive supply of the control IC.
This is the reference output. It provides charging current for capacitor C
T
through resistor R
T
.
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 (V
OH
) 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 to back
to the power source ground.
No connection (14−pin package only). These pins are not internally connected.
−
11
V
C
−
−
9
2,4,6,13
GND
NC
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