POWER MANAGEMENT
1www.semtech.com
SC4508
Buck or Buck-Boost(Inverting)
Current Mode Controller
Description
Revision: November 30, 2004
Features
Typical Application Circuits
Applications
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
MLPQ-12 package
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
The SC4508 is a low voltage current mode switching regu-
lator controller that drives a P-channel power MOSFET
with programmable switching frequency. It can be con-
figured in either buck or buck boost (inverting) convert-
ers. 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. The output
voltage can adjusted by external resistor divider. The
switching frequency is programmable up to 1.5MHz,
allowing small inductor and capacitor values to minimize
PCB space. The operating current level is programmable
via an external sense resistor. Accessible reference volt-
age allows users to make output voltage as low as they
want.
VREF
CS
OUT
VDD
FB+
FB-
COMP
SS/SHDN OSC
SC4508
Vin
GND
CS
OSC
VDD
Vin
SS/SHDN
FB-
GND
SC4508
OUT
FB+
VREF
COMP
2 2004 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4508
Electrical Characteristics
Absolute Maximum Rating
Unless specified: VDD = 2.7V to 15V , TA = TJ = -40°C to 125°C
retemaraPlobmySmumixaMstinU
DNGotDDV 61ot3.0-V
DNGotNDHS/SS 2.3V
DNGotCSO,PMOC,-BF,+BF 5V
tnerruCFERV 1Am
kniSroecruoSTUO 052Am
tneibmAotnoitcnuJ,ecnatsiseRlamrehT θ
AJ
84W/C°
esaCotnoitcnuJ,ecnatsiseRlamrehT θ
CJ
3W/C°
egnaRerutarepmeTegarotST
GTS
051+ot06-C°
egnaRerutarepmeTnoitcnuJT
J
051+ot04-C°
s04-01erutarepmeTwolfeRRIkaePT
GKP
062C°
retemaraPsnoitidnoCtseTniMpyTxaMtinU
ylppuSrewoP
egnaRegatloVtupnI 7.251V
tnerruCtnecseiuQwol=NDHS002005Aµ
tnerruCgnitarepOdaoloN,hgih=NDHS3Am
tuokcoLegatlovrednU
dlohserhTtratSV
DD
gnisir5.255.2V
siseretsyHOLVU 001Vm
rotallicsO
egnaRycneuqerF 0010051zHK
ycneuqerFC
T
Fp003=054005055zHK
tnerruCegrahC 001Aµ
reifilpmArorrE
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tnerruCsaiBtupnI 001An
ecnatcudnocsnarT 5Sm
tnerruCkniSroecruoStuptuO 05001Aµ
Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters
specified in the Electrical Characteristics section is not implied. Exposure to Absolute Maximum rated conditions for extended periods of time may
affect device reliability.
3
2004 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4508
Electrical Characteristics
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rotarapmoCMWP
tuptuOotyaleD 05sn
ecnerefeRFERV
egatloVtuptuO 132.152.1962.1V
tnerruCtuptuO 1Am
noitalugeReniLV
DD
I,V51ot7.2=
FERV
Am1=4Vm
noitalugeRdaoLV
DD
I,V5=
FERV
Am1ot0=4Vm
nwodtuhS/tratStfoS
tnerruCegrahC 02Aµ
tnerruCegrahcsiD 21Am
egatloVthgiHcigoLNDHS 2V
egatloVwoLcigoLNDHS 52.0V
timiLtnerruC
dlohserhTelcyCybelcyCV
DD
V5=09011031Vm
dlohserhTnwodtuhS 61.0V
tuptuOotyaleD 05Sn
tuptuO
)H(ecnatsiseR-nOevirDetaGV
DD
I,V5=
GNICRUOS
Am052=02mhO
)L(ecnatsiseR-nOevirDetaGV
DD
I,V5=
KNIS
Am052=02mhO
emiTesiRC
TUO
Fp002=02Sn
emiTllaFC
TUO
Fp002=02Sn
Unless specified: VDD = 2.7V to 15V , TA = TJ = -40°C to 125°C
Note:
(1) This device is ESD sensitive. Use of standard ESD handling precautions is required.
4 2004 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4508
Pin Descriptions
Notes:
(1) Only available in tape and reel packaging. A reel
contains 3000 devices.
(2) Lead free product. This product is fully WEEE and
RoHS compliant.
ECIVEDEGAKCAPT(egnaR.pmeT
J
)
TRTLMI8054CS
)2(
21-QPLM
)1(
C°051otC°04-
Ordering Information
Pin Configuration
TOP VIEW
(MLPQ-12 4x4)
1OUT
2CS
3SS/SHDN
9 FB+
8 FB-
7 PGND
4
VREF
5
NC
6
AGND
12
VDD
11 10
OSC COMP
#niPemaNniPnoitcnuFniP
1TUO .DNGPotDDVmorfsgniwsTUO.TEFSOM-PlanretxeroftuptuorevirdetaG
2SC .SCdnaDDVneewtebrotsiseresnestnerrucatcennoC.niptupniesnestnerruC
3NDHS/SS pupmarehT.DNGAdnanipsihtneewtebroticapaclanretxenastcennoC.niptratstfoS
dellupsiNDHS/SSVnehwnwodtuhsotniseogecivedehT.roticapacehtybdenifedsiemit
.V52.0woleb
4FERV cimarecFu1.0ahtiwssapyB.Am1otpuecruosnacFERV.tuptuoecnereferV52.1
.DNGAotFERVmorfroticapac
5CN.noitcennocoN
6DNGA.dnuorggolanA
7DNGP.dnuorgrewoP
8-BF.tupnignitrevnireifilpmarorrE
9+BF.tupnignitrevni-nonreifilpmarorrE
01PMOC pooltcennoC.reifilpmarorreecnatcudnocsnartlanretniehtrofnipnoitasnepmoC
.DNGAotPMOCmorfkrowtennoitasnepmoc
11CSO ehttesotDNGAotCSOmorfroticapaccimarecatcennoC.tupnitesycneuqerfrotallicsO
ehttesotnoitauqeesU.zHM5.1otzHK001morfycneuqerfrotallicsolanretni
.DNGAotCSOmorfroticapacehtsiC.ycneuqerfrotallicso
21DDV .DNGPotDDVmorfroticapaccimarecFu1assapyB.egatlovylppuS
-LAMREHT
DAP
detcennoctoN.saivelpitlumgnisuenalpdnuorgottcennoC.sesoprupgniknistaehrofdaP
.yllanretni
65.0C
A100
f•µ
=
5
2004 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4508
Block Diagram
Marking Information
Top View
yyww = Date Code (Example: 0012)
6 2004 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4508
Application Information
The SC4508 is designed to control buck (step down) or
buck-boost (inverting) converter with P-channel MOSFET
as a switch using current mode, programmable switching
frequency architecture. During steady state operation,
the switch is turned on each cycle and turned off when
the voltage across current sense resistor exceeds the
voltage level at COMP pin set by voltage loop error
amplifier. A fixed 0.5V artificial ramp is added internally
to the sensed current signal for operations when duty-
cycle is larger than 50%. In over load or output shortage
condition, if the sensed current signal reaching typical
120mV, the switch is turned off immediately in the same
cycle. If the sensed current signal further increases to
typical 160mV, not only the switch is turned off but also
the soft start capacitor is discharged by a internal MOSFET
to ground then charging back to threshold 250mV during
which the switch is held off. With the “hiccup” mode over
current protection, the thermal stress is reduced in the
faulty conditions.
Current Sense and Current Limit
The typical cycle-by-cycle current limit threshold in the
current sense pin of the SC4508 is 120mV. The over
current limit is assumed typical 120% of full load current.
Then the current sense resistor can be calculated by the
following equation:
Rs = )pk(I%120
mV120
L
•
IL (pk) = )
VV
VV
(
Lf
VV
2
1
I
DIN
DO
s
OIN
O+
+
•
−
+ for Buck
BoostBuckfor
)
VVV
VV
(
Lf
V
2
1
V
VVV
I)pk(I
DOIN
DO
s
IN
IN
DOIN
OL
−++ +
•
+
++
=
IO - full load current
VO - output voltage
VIN - input voltage
VD - diode forward voltage drop
fS - switching frequency
L - inductor
Inductor Selection
A inductor is chosen based on the required output ripple
current which is usually 20-30% of load current. The ripple
current ∆I decreases with higher inductance and
increases with higher input voltage VIN or output voltage
VO.
)
VV
VV
(
If
VV
L
DIN
DO
Ls
OIN +
+
∆•−
= for Buck
)
VVV
VV
(
If
V
L
DOIN
DO
Ls
IN ++ +
ƥ
= for Buck-Boost
Specify the maximum inductor current larger than IL(pk)
set by the current sense resistor RS to avoid the inductor
core saturation.
Input Capacitor CIN and Output Capacitor COUT Selection
Both input and output capacitors need to be sized to
handle the ripple current safely. Buck converters have
high ripple current in the input side while buck-boost
converters have high ripple current in the both input and
output capacitors. Therefore, the RMS value of the
current must be less than the high frequency ripple rating
of the capacitors. In continuous mode operation,
IN
OINO
OCAP_INPUT_RMS V
)VV(V
II −
≈ for Buck
IN
DO
OCAP_RMS V
VV
II +
≈ for Buck-Boost input and output
capacitors.
The C
OUT
is selected for Buck converter is by required
output ripple voltage and converter loop stability. The
output ripple is determined by:
)
Cf8
1
ESR(IV
OUTs
LO +∆≈∆
where f
S
is converted switching frequency, C
OUT
is output
capacitance and ∆I
L
is inductor ripple current.
7
2004 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4508
Application Information (Cont.)
In Buck converter, the worst case input RMS ripple current
occurs at 50% duty cycle, or at VIN = 2VOUT. Under this
condition, the input capacitors RMS ripple current is about
half of the load current.
MOSFET Selection
The selection criteria for the power MOSFET is its
operating junction temperature not exceeding the
maximum junction temperature. Therefore, from a
specified ambient temperature, the maximum junction
to ambient temperature rise has to be determined, which
is related to the MOSFET power dissipation or power loss.
The power loss includes conduction loss and switching
loss. The conduction loss is given by:
)ON(DS
2
O
DIN
DO
ON RI)
VV
VV
(P +
+
= for Buck
)ON(DS
2
ODO
IN
DOIN
ON RI)VV)(
V
VVV
(P +
++
= for Buck-Boost
Freewheeling Diode Selection
The Schottky diode is recommended as freewheeling
diode in the both Buck and Buck-Boost applications. The
diode conducts during the off-time. The diode voltage
and current ratings are selected based upon the peak
reverse voltage, the peak current and average power
dissipation.
Buckfor
VV
VV
II,
2
I
II,VV
DIN
OIN
O)AVG(D
L
O)PEAK(DIN)REV(D +
−
=
∆
+==
BoostBuckfor
II,
2
I
)
V
VVV
(II,VVV O)AVG(D
L
IN
DOIN
O)PEAK(DOIN)REV(D
−
=
∆
+
++
=+=
The most stressful condition for the diode occurs when
the output is shorted. Under this condition, due to the
VOUT = 0, the diode conducts at close to 100% duty cycle.
Therefore, attention should be paid to the thermal
condition when laying out a board.
LAYOUT GUIDELINES
Careful attention to layout requirements are necessary
for successful implementation of the SC4508 PWM con-
troller. High switching currents are present in the appli-
cation and their effect on ground plane voltage differen-
tials must be understood and minimized.
1). The high power parts of the circuit should be laid out
first. A ground plane should be used, the number and
position of ground plane interruptions should be such as
to not unnecessarily compromise ground plane integrity.
Isolated or semi-isolated areas of the ground plane may
be deliberately introduced to constrain ground currents
to particular areas, for example, the input capacitor and
output capacitor ground.
2). The loop formed by the Input Capacitor(s) (Cin), the
MOSFET and the Schottky diode or inductor must be kept
as small as possible. This loop contains all the high cur-
rent, fast transition switching. Connections should be as
wide and as short as possible to minimize loop induc-
tance
3). The connection between the junction of MOSFET,
Schottky diode and the output inductor should be a wide
trace or copper region. It should be as short as practical.
Since this connection has fast voltage transitions, keep-
ing this connection short will minimize EMI.
4) The Output Capacitor(s) (Cout) should be located as
close to the load as possible, fast transient load cur-
rents are supplied by Cout only, and connections between
Cout and the load must be short, wide copper areas to
minimize inductance and resistance.
5) A separate analog ground plane connects to the
SC4508 AGND pin. All analog grounding path including
decoupling capacitors, feedback resistors, compensation
components, and current-limit setting resistors should
be connected to this plane.
8 2004 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4508
Evaluation Board Schematic, Buck
Bill of Materials
metIytitnauQecnerefeRtraPrerutcafunaM
12 2C,1CV61/Fu74M74BPT61:N/PoynaS
213CFn1
31,4CFu1
42 7C,5CFu1.0
516CFp033
62 9C,8CV3.6/Fu074M074BPT61:N/PoynaS
7101CFn2
8111CFn7.2
911LHu01
0111R90.0
1112R01
2114RK02
3115RK57.4
4116RK42.1
5117RK01
6111U8054CS.proChcetmeS
7112UA201P2SFDF:N/PdlihcriaFA201P2SFDF
GND
12V
GND
TP4
TP3
TP8
1
2
3
4 5
6
7
8
U2 FDFS2P102A
R1
0.09
C5
0.1uF
R7
10K
C4
1uF
R5
3.74K
C1
47uF/16V
C8
470uF/6.3V
L1
10uH
TP1
TP7
COMP 10
NC
5
SS/SHDN
3
OSC
11
FB+ 9
FB- 8
VREF 4
OUT 1
CS 2
VDD
12
AGND
6
PGND
7
U1
SC4508
U1
TP6
R6
1.24K
R4
20K
TP5
TP2
C11
2.7nF
C9
470uF/6.3V
C3
1nF
C2
47uF/16V
C7
0.1uF
C6
330pF
R2 10
C10
2nF
fs=440KHz
6V/1
A
9
2004 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4508
Evaluation Board Schematic, Buck-Boost
metIytitnauQecnerefeRtraPrerutcafunaM
13 8C,2C,1CV61/Fu74M74BPT61:N/PoynaS
213CFn1
314CFu1
42 7C,5CFu1.0
516CFp086
619CV3.6/Fu001
7101CFp001
8111CFn7.2
916DWH9185N1
0111LHu3.3
1111R1.0
2112R001
3114RK02
4115RK40.6
512 8R,6R942
6117RK01
7119RK0.1
9111U8054CS.proChcetmeS
0212UYD1384iSyahsiV
Bill of Materials
GND
C8
47uF/16V
R1
0.1
C6
680pF
TP8
TP1
C4
1uF
L1
33uH
TP7TP6
C10
100pF
1
2
3
4 5
6
7
8
U2 Si4831DY
C2
47uF/16V
C11
2.7nF
C1
47uF/16V
C5
0.1uF
TP3
R5
6.04K
D1
1N5819HW
C7
0.1uF
COMP 10
NC
5
SS/SHD N
3
OSC
11
FB+ 9
FB- 8
VREF 4
OUT 1
CS 2
VDD
12
AGND
6
PGND
7
U1
SC4508
U1
R2 100
R6 240
R7
10K
TP5
C3
1nF
TP4
R4
20K
C9
100uF/6.3V
TP10
fs=250KH z
R8
249
R9 1.0K
TP9
12V
GND
-5V/0.1
A
10 2004 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4508
Contact Information
Semtech Corporation
Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805)498-2111 FAX (805)498-3804
Outline Drawing - MLPQ-12, 4 x 4
0.25
0.00
-
0.30
-
A D
E
B
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
1.
2.
NOTES:
-
--
-
INDICATOR
(LASER MARK)
PIN 1
NOM
DIMENSIONS
INCHES
aaa
bbb
D1
E1
A2
A1
DIM
D
L
N
e
E
MIN
b
A
MAX
MILLIMETERS
MINMAX NOM
4.104.003.90.161.157.153
.153 .157 .161 3.90 4.00 4.10
SEATING
PLANE
bbb C A B
E1
aaa C
e
A
A1
D/2
1
A2
C
bxN
LxN
D1
N
2
E/2
0.08.003
.004 0.10
.010
.074
.000
.031
.018
.074
-
.031 BSC
.085
12
.022
.085
.089
.026
.089
(.008)
.012
.040
.014
.002
-
0.35
0.05
1.00
2.25
2.25
0.65
0.80 BSC
1.90
0.45
1.90
2.15
0.55
12
2.15
(0.20)
0.80
P
Y
Z
P
Y
X
H
K
K
DIM
C
.091
.091
2.30
2.30
DIMENSIONS
MILLIMETERSINCHES
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
NOTES:
1.
.031
.189
.041
.016
.106
(.148)
0.80
4.80
1.05
0.40
(3.75)
2.70
G
H
2x (C) 2x G 2x Z
X
Land Pattern - MLPQ-12, 4 x 4
