sc4808b-1 Semtech Corporation, sc4808b-1 Datasheet - Page 11

• Manufacturer Part Number: sc4808b-1
• Description: Sc4808b-1 High-performance Dual Ended Pwm Controller
• Manufacturer: Semtech Corporation

VCC UNDER VOLTAGE LOCK OUT
Depending on the application and the voltages available,
the SC4808B-1 (UVLO = 4.4V) can be used to provide the
VCC undervoltage lock out function to ensure the convert-
ers controlled start up.
Before the VCC UVLO has been reached, the internal refer-
ence, oscillator, OUTA/OUTB drivers, and logic are disabled.
LINE UNDER VOLTAGE LOCK OUT
The SC4808B-1 also provides a line undervoltage (LUVLO
= Vref) function. The LUVLO pin is programmed via an ex-
ternal resistor divider connected as shown below. The ac-
tual start-up voltage can be calculated by using the equa-
tion below:
REFERENCE
A 3.125V(SC4808B-1) reference voltage is available that
can be used to source a typical current of 5mA to the ex-
ternal circuitry. The Vref can be used to provide the oscilla-
tor RC network with a regulated bias.
POWER MANAGEMENT
V
Application Information (Cont.)
2006 Semtech Corp.
Startup
15
R26
2.2k
C29
82p
R24 10k
R25 18
V
REF
R
R27
15k
C31
200p
REF
23
R
33
5
4
3
2
1
R
SYNC
REF
FB
CS
RC
SYNC
33
Vin
SC4808
2.2u,16V
U4
C26
LUVLO
OUTB
OUTA
GND
VCC
56.2k
R23
6
7
8
9
10
10k
R33
VCC
C33
0.1u,25V
R28
10
11
OSCILLATOR
The oscillator frequency is set by connecting a RC network
as shown below.
The oscillator has a ramp voltage of about Vref/2. The os-
cillator frequency is twice the frequency of the OUTA and
OUTB gate drive controls.
The oscillator capacitor C31 is charged by a current sourced
from the Vref through R27. Once the RC pin reaches about
Vref/2, the capacitor is discharged internally by the
SC4808B-1. It should be noted that larger capacitor val-
ues will result in a longer dead time during the down slope
of the ramp.
The following equation can be used as an approximation
of the oscillator frequency and the Dead time:
where:
The recommended range of timing resistors is between 10
kohm and 200kohm, range of timing capacitors is between
100pF and 1000pF. Timing resistors less than 10 kohm
should be avoided.
T
C
C
F
deadtime
OSC
TOT
SC
4808
_
A
C
OSC
22
R
C
OSC
OSC
pF
C
C
3
TOT
1
SC
V
10
REF
4808
R27
15k
C31
200p
3
REF
0
8 .
0
C
5 .
Circuit
5
4
3
2
1
SYNC
F
REF
FB
CS
RC
SYNC
OSC
Vin
SC4808
2.2u,16V
_
U4
B
C26
LUVLO
OUTB
OUTA
GND
VCC
56.2k
R23
R
OSC
6
7
8
9
10
C
SC4808B-1
TOT
1
10k
R33
VCC
C33
0.1u,25V
0
9 .
www.semtech.com
R28
10
0
0