SC4502 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC4502' PDF : 19 Pages View PDF
SC4502/SC4502H
POWER MANAGEMENT
Applications Information
IN
6/8
I HYS
4.6 µA
R3
SWITCH CLOSED
WHEN Y = “1”
SHDN
3
+
Y
R4
1.1V
-
COMPARATOR
VL = VH − VHYS = 2.75V − 0.69V = 2.06V > 1.4V .
Frequency Compensation
Figure 7 shows the equivalent circuit of a boost converter
using the SC4502/SC4502H. The output filter capacitor
and the load form an output pole at frequency:
ωp2
=
2 ⋅ IOUT
VOUT ⋅ C2
=
2
ROUT ⋅ C2
(13)
where
C2
is
the
output
capacitance
and
ROUT
=
VOUT
IOUT
is
the equivalent load resistance.
SC4502/SC4502H
Figure 6. Programmable Hysteretic UVLO Circuit
The zero formed by C2 and its equivalent series resistance
(ESR) is neglected due to low ESR of the ceramic output
capacitor.
with VL > 1.4V .
There is also a right half plane (RHP) zero with angular
frequency:
Example: Increase the turn on voltage of a VIN = 3.3V boost
converter from 1.4V to 2.75V.
( ) ωZ2
=
ROUT
⋅ 1−D
L
2
(14)
Using VH = 2.75V and R4 = 100KΩ in (12),
R3 = 150KΩ .
ωz2 decreases with increasing duty cycle D and increasing
IOUT. Using the 5V to 12V boost regulator (1.4MHz) in
Figure 1(a) as an example,
The resulting UVLO hysteresis is:
VHYS = IHYSR3 = 4.6µA • 150KΩ = 0.69V
ROUT
≥
5V
0.5A
= 10Ω
The turn off voltage is:
VIN
POWER
STAGE
COMP
R3
C6
C4
-
FB
Gm
+
RO
1.242V
VOLTAGE
REFERENCE
C5 R1
IOUT
VOUT
ESR
C2
ROUT
R2
2005 Semtech Corp.
Figure 7. Simplified Block Diagram of a Boost Converter
12
www.semtech.com
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