Datasheet

4
UCC1581
UCC2581
UCC3581
The UCC3581’s oscillator allows the user the flexibility to
program the frequency and the duty cycle by adjusting
two resistors and a capacitor. Application Diagram Fig. 1
shows these components as RT1, RT2, and C
T. RT1 pro
-
grams the timing capacitor charging current which results
in a linear ramp charging C
T. Discharge of CT is accom
-
plished though RT2 which results in a standard RC dis
-
charge waveform. The oscillator on-time (C
T charging) is
calculated by the formula
tRTC
ON T
=••0082 1.
.
The off-time (C
T discharging) is calculated by the formula
tRTC
OFF T
=••095 1.
.
Resistor RT1 programs the charging current. The current
is:
2.0V
RT 1
.
CT charging current is 9.2 times the current in RT1. RT1
can range from 220kΩ to 1MΩ. Minimum capacitor size
is 100pF, and minimum RT2 size is 10k.
A Block Diagram of the Oscillator is shown in Fig. 2. The
oscillator also has an external synchronization pin.
When a low to high level is detected, and if the oscilla-
tor’s output is in the high state (C
T charging), the oscilla-
tor output immediately goes low and C
T starts
discharging. The sync input is rising edge sensitive and
is ignored when the oscillator output is low.
APPLICATION INFORMATION
1
3
2
14
C
T
RT2
V
IN
1µF
D2
4
Q1
5
6
1µF
7
T1
R
L
REF &
E/A
U1
CT
GT
VDD
OUT
GND
REF
ISEN
V
IN
8VC
9DCMIN
10SS
11EN
U1
REF
REF
C
SS
12RT1
13SYNC
RT2
RT1
REF
UCC3581
D1
BSS129 OR
EQUIV.
Figure 1. Application diagram.
UDG-99043
Figure 2. Oscillator.
UDG-96105