UC3844, UC3845, UC2844, UC2845
25
V
= 30 V
C = 15 pF
CC
L
T = 255C
20
15
10
5
A
R = 10 k
T
C = 3.3 nF
T
V
FB
= 0 V
I
Sense
T = 255C
= 0 V
A
0
0
10
20
30
40
100 ns/DIV
V
CC
, SUPPLY VOLTAGE (V)
Figure 14. Output Cross Conduction
Figure 15. Supply Current versus
Supply Voltage
PIN FUNCTION DESCRIPTION
Pin
8−Pin
14−Pin
Function
Description
1
2
1
3
Compensation
This pin is Error Amplifier output and is made available for loop compensation.
Voltage
Feedback
This is the inverting input of the Error Amplifier. It is normally connected to the switching
power supply output through a resistor divider.
3
4
5
7
Current Sense
A voltage proportional to inductor current is connected to this input. The PWM uses this
information to terminate the output switch conduction.
R /C
The Oscillator frequency and maximum Output duty cycle are programmed by connecting
T
T
resistor R to V and capacitor C to ground. Operation to 1.0 MHz is possible.
T
ref
T
5
6
−
GND
This pin is combined control circuitry and power ground (8−pin package only).
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. The output switches at one−half the oscillator frequency.
7
8
−
12
14
8
V
This pin is the positive supply of the control IC.
CC
V
This is the reference output. It provides charging current for capacitor C through resistor R .
ref
T
T
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
V
C
The Output high state (V ) 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.
OH
−
−
9
GND
NC
This pin is the control circuitry ground return (14−pin package only) and is connected to back
to the power source ground.
2,4,6,13
No connection (14−pin package only). These pins are not internally connected.
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