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PDF SC4508A Data sheet ( Hoja de datos )

Número de pieza SC4508A
Descripción Buck or Buck-Boost Current Mode Controller
Fabricantes Semtech Corporation 
Logotipo Semtech Corporation Logotipo



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SC4508A
Buck or Buck-Boost (Inverting)
Current Mode Controller
POWER MANAGEMENT
Description
The SC4508A is a low voltage current mode switching
regulator controller that drives a P-channel power
MOSFET with programmable switching frequency. It can
be configured in either buck or buck boost (inverting) con-
verters. The converters can be operated from 2.7V to
15V input voltage range. The typical operating supply
current is 3mA and a shutdown pin allows the user to
turn the controller off reducing it to less than 200µA.
www.DataTShheeeto4Uu.tcpoumt voltage can adjusted by external resistor di-
vider. The switching frequency is programmable up to
1.5MHz, allowing small inductor and capacitor values to
minimize PCB space. The operating current level is pro-
grammable via an external sense resistor. Accessible ref-
erence voltage allows users to make output voltage as
low as they want.
Features
‹ Wide input voltage range 2.7V to 15V
‹ Programmable output voltage
‹ Programmable switching frequency up to 1.5MHz
‹ Buck or buck boost (inverting) configuration
‹ Current mode control with slope compensation
‹ Very low quiescent current in shutdown mode
‹ Accessible reference voltage
‹ Hiccup mode after 32 cycle-by-cycle OCP
‹ 4mm x 4mm MLPQ-12 lead free package. This
product is fully WEEE and RoHS compliant
Applications
‹ Low power point of use converters
‹ Single or multiple output low power converters
‹ Positive and/or negative output voltage
‹ DSL cards
‹ Graphic cards
‹ I/O cards
‹ Negative bias supplies
Typical Application Circuits
Vin Vin
VREF
V+
FB+
FB-
COMP
SC4508A
CS
OUT
SS/EN
GND OSC
Vout
VREF
V+
FB+
FB-
COMP
SC4508A
CS
OUT
SS/EN
GND OSC
Vout
Revision: July 25, 2007
1
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1 page




SC4508A pdf
POWER MANAGEMENT
Block Diagram
www.DataSheet4U.com
SC4508A
Marking Information
Top View
yyww = Date Code (Example: 0012)
2007 Semtech Corp.
5
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SC4508A arduino
SC4508A
POWER MANAGEMENT
Application Information (Cont.)
50
40
Cg(100, Rds)
Cg(200, Rds)
Cg(500, Rds) 20
www.DataSheet4U.com
1 00
1
5 10 15
Rds
On-resistance (mOhm)
FOM:100*10^{-12}
FOM:200*10^{-12}
FOM:500*10^{-12}
20
20
Figure 5. Figure of Merit curves.
manufacturers’ data sheet. From the FDS6675
datasheet, Rds(on) is less than 14mwhen Vgs is greater
than 10V. However Rds(on) increases by 30% as the
junction temperature increases from 25oC to 110oC.
The switching losses can be estimated using the simple
formula:
Pts
=
1
2
(t
r
+
tf )(1+
δ
2
)Idc
Vinfs
where tr is the rise time and tf is the fall time of the
switching process. Different manufacturers have
different definitions and
clarify these, we sketch
test
the
ctyopnidciatiloMnsOfSoFrEtTr
asnwditctfh.inTog
characteristics under clamped inductive mode in Figure
6.
The closer the curve is to the origin, the lower is the FOM.
This means lower switching loss or lower conduction loss
or both. It may be difficult to find MOSFETs with both low
CCgg
and
has
ltoowbeRdms(oan.deU.sually
a
trade-off
between
Rds(on
and
MOSFET selection also depends on applications. In many
applications, either switching loss or conduction loss
dominates for a particular MOSFET. For buck and buck-
boost converters with high input to output voltage ratios,
the MOSFET is hard switched but conducts with very low
duty cycle. For such applications, MOSFETs with low Cg
should be used.
MOSFET power dissipation consists of:
a) conduction loss due to the channel resistance Rds(on),
b) switching loss due to the switch rise time tr and fall
time tf, and
c) the gate loss due to the gate resistance RG.
The RMS value of the MOSFET switch current is calculated
as:
IQrms = Idc
D(1 +
δ2
12
)
The conduction losses are then
Ptc =
I2
Qrms
Rds(on)
Idc is average inductor current. In buck converter, it is
also load current. In buck-boost, it is load current divided
by 1-D.
Figure 6. MOSFET switching characteristics
In Figure 6,
Qgs1 is the gate charge needed to bring the gate-to-source
voltage Vgs to the threshold voltage Vgs_th,
Qgs2 is the additional gate charge required for the switch
current to reach its full-scale value
Qgd is the charge needed to charge
Igdas.taen-tdo-drain
(Miller)
capacitance when Vds is falling.
Switching losses occur during the time interval [t1, t3].
Defining tr = t3-t1 and tr can be approximated as:
tr
=
(Qgs2 + Qgd )Rgt
Vcc Vgsp
where Rgt is the total resistance from the driver supply
rail to the gate of the MOSFET. It includes the gate driver
internal impedance Rgi, external resistance Rge and the
gate resistance Rg within the MOSFET i.e.
Rgt = Rgi+Rge+Rg
Rds(on) varies with temperature and gate-source voltage.
Curves showing Rds(on) variations can be found in
Vgsp is the Miller plateau voltage shown in Figure 11.
Similarly an approximate expression for tf is:
2007 Semtech Corp.
11
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