Datasheet

 
   

SLUS431A – MAY 2000 – REVISED DECEMBER 2000
8
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
pin descriptions (continued)
UNDER–VOLTAGE
OVER–VOLTAGE
VOLT–SEC
LMT
SOFT
START
STOP
START
14
1
+
1 V
+
1 V
ISS
8
Q
QS
R
S/D
LATCH
CT
4 V
4 V
4
5 V
5 V
3
5
7
2 ISS
S/S
PWM
SYNC MAX D/C
RAMP CLK
OSCILL
Q
Q
S
R
PWM
LATCH
CURRENT
LIMIT
SHUT DOWN
SD
PWM DRIVE
1.375 V 10
11
9
1.0V
4V
2
ISD
10
ISD
1 V
REF
GEN
5.0 V
UVLO
17 V
6
10 / 8 V
12 V
12
13
+
VDD REG
UVS
OVS
VS
FB
SS
RT
AGND
SD
PGND
CS
OUT
VREF
VDD
START
HI
HI
LO
LO
5V
UDG–00160
Figure 1. Block Diagram
DETAILED DESCRIPTION
start-up oscillator
The RT pin is connected to an internal 2.0 V nominal, unity-gain closed-loop amplifier that is referenced to a
voltage divider off the 5.0-V reference. When a 22.2-kΩ resistor is connected from the RT pin to GND an
approximate I
RT
= 90µA internal current is realized that charges the internal oscillator capacitor that is
approximately 58 pF. I
RT
= 90µA produces a maximum free running oscillator frequency of 360 KHz at a 72%
duty cycle. When a 133.3-kΩ resistor is connected from the RT pin to GND an approximate I
RT
= 15 mA internal
current is realized that produces a minimum free running oscillator frequency of 60 KHz at approximately 72%
duty cycle. This maximum duty cycle is setup by on-chip MOSFET current mirrors that are not programmable.
Any frequency between these two limits (6:1 maximum to minimum frequency) is obtainable by linearly scaling
the RT resistance between the minimum 22.2-kΩ and maximum 133.3-kΩ values. The secondary-side PWM
frequency should be fixed at (1 / 0.9) or 1.11 times the user programmed primary-side free running oscillator
frequency for proper primary-side synchronization. Therefore, in all cases the recommended secondary-side
synchronization frequency shall be 1.11 times higher than the selected primary-side free running startup
frequency. Taking into consideration the two extreme limits for primary-side free running startup frequency, the
secondary-side operating frequency should be set between 400 kHz and 67 kHz.