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PDF SC2618 Data sheet ( Hoja de datos )
| Número de pieza | SC2618 | |
| Descripción | Miniature PWM Controller for Buck Regulator | |
| Fabricantes | Semtech | |
| Logotipo |  |
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POWER MANAGEMENT
Description
The SC2618 is a hysteretic mode PWM controller de-
signed for high efficiency, low cost “Point of Load” appli-
cations.
The hysteretic control scheme allows the use of ceramic
output capacitors without the need for compensation
components and difficult calculations.
An internal soft start prevents output voltage from start
up overshoot. A “Hiccup” short circuit protection provides
protection against output short circuits as well as ad-
verse input supply sequencing.
SC2618
Miniature PWM Controller
for Bwwuwc.DkataRSheeetg4Uu.cloamtor
Features
Operating Frequency up to 500kHz
Input supply from 5V to 12V
0.5A gate drive capability
Internal soft start
Internal, trimmed bandgap reference (±1%)
Hiccup mode short circuit protection
Input supply sequence protection
6 lead SOT23 package
Applications
Graphics IC Power supplies
Embedded, low cost, high efficiency converters
Typical Application Circuit
VIN
BOOST
VCC
GND
Q1
5 BST DH 6
3 VCC DL 1
2 GND FB 4
SC2618
Q2
VO
GND
Revision: September 19, 2005
1
www.semtech.com
1 page  
SC2618
POWER MANAGEMENT
Component Selection
www.DataSheet4U.com
General design guideline of switching power supplies can
be applied to the component selection for SC2618.
Output Filter Components
The purpose of soft start is to control inductor current
during start up, this in turn controls the amount of ex-
cess charge the output capacitor will receive and there-
fore the output voltage overshoot that will occur at the
end of soft start.
VIN
Inductor and MOSFETs
The selection of inductor and MOSFETs should meet ther-
mal requirement because they are power loss dominant
components. Pick an inductor with as high inductance
as possible if without adding extra cost and size. Higher
inductance, lower ripple current, smaller core loss and
higher efficiency. However, too high inductance slows
down output transient response. It is recommended to
choose the inductance that gives the inductor ripple cur-
rent to be approximate 20% of maximum load current.
So choose inductor value from:
Q1
L VO
Q2
GND
Co
GND
2618-02
Fig. 2. Output filter components
L
=
IO
5
⋅ fosc
⋅VO
⋅ (1− VO
VIN
)
The MOSFETs are selected from their Rdson, gate charge,
and package. The SC2618 provides 0.5A gate drive cur-
rent. To drive a 25nC gate charge MOSFET gives 25nC/
0.5A=50ns switching time. The switching time ts contrib-
utes to the top MOSFET switching loss:
VO VP
vC
PS = IO ⋅VIN ⋅ tS ⋅ fOSC
There is no switching loss for the bottom MOSFET be-
VREF cause of its zero voltage switching. The conduction losses
of the top and bottom MOSFETs are given by:
0
PC _ TOP
=
I
2
O
⋅ Rdson
⋅D
IL
iL
PC _ BOT
=
I
2
O
⋅ Rdson ⋅ (1−
D)
0 If the requirement of total power losses for each MOSFET
t tSS tP
Fig. 3. Soft start, inductor current and overshoot
is given, the above equations can be used to calculate
the values of Rdson and gate charge can be calculated
using above equations, then the devices can be deter-
mined accordingly. The solution should ensure the
Normally, controllers with an external soft start capaci-
tor, have soft start set so long that overshoot is unde-
MOSFET is within its maximum junction temperature at
highest ambient temperature.
tectable. Since the soft start for the SC2618 is internal,
it is necessarily limited and the maximum values of L and
Co should meet the constraints of:
Output Capacitor(s)
The output capacitors should be selected to meet both
output ripple and transient response criteria. The output
L ⋅ CO
=
t
2
SS
⋅
XS
50
where tss is soft start time, 100us typically, and Xs is the
capacitor ESR causes output ripple VRIPPLE during the
inductor ripple current flowing in. To meet output ripple
criteria, the ESR value should be:
percentage overshoot allowable.
RESR
<
L ⋅ fOSC ⋅VRIPPLE
VO
⋅ (1−
VO
VIN
)
2004 Semtech Corp.
5
www.semtech.com
5 Page | ||
| Páginas | Total 9 Páginas | |
| PDF Descargar | [ Datasheet SC2618.PDF ] | |
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