SC4808B-1 데이터 시트보기 (PDF) - Semtech Corporation
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SC4808B-1 Datasheet PDF : 28 Pages
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SC4808B-1
POWER MANAGEMENT
Application Information (Cont.)
VCC UNDER VOLTAGE LOCK OUT
OSCILLATOR
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.
The oscillator frequency is set by connecting a RC network
as shown below.
REF
C26
2.2u,16V
VCC
0
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:
R27
15k
C31
200p
U4
5 REF
GND 6
4 FB
OUTB 7
3 CS
OUTA 8
2 RC
VCC 9
1 SYNC
LUVLO 10
R28
10
C33
0.1u,25V
SC4808
0
SYNC
56.2k
10k
( ) VStartup = VREF ×
R23 + R33
R33
Vin
R23
R33
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.
R24 10k
15
R25 18
R26
2.2k
C29
82p
REF
C26
2.2u,16V
VCC
R27
15k
C31
200p
U4
5 REF
GND 6
4 FB
OUTB 7
3 CS
OUTA 8
2 RC
VCC 9
1 SYNC
LUVLO 10
R28
10
C33
0.1u,25V
SC4808
SYNC
56.2k
10k
Vin
R23
R33
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:
FOSC _ A
≅
1
ROSC CTOT
× 0.8
FOSC _ B
≅
1
ROSC CTOT
× 0.9
where:
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.
CTOT = COSC + CSC 4808 + CCircuit
CSC 4808 ≅ 22 pF
Tdeadtime
≅
COSC ×VREF × 0.5
3 ⋅10−3
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.
2006 Semtech Corp.
11
www.semtech.com
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