POWER MANAGEMENT
1www.semtech.com
SC2612A/B/C
600kHz/1MHz/200kHz
Step-Down DC/DC Converter
Features
Applications
Revision 1, January 2003
Typical Application Circuit
Description
The SC2612 is a voltage mode switcher designed for low
cost, “point of use” voltage conversion. SC2612 is avail-
able with fixed switching frequencies of 200kHz
(SC2612C), 600kHz (SC2612A) and 1MHz (SC2612B).
The SC2612 has soft start and enable functions and is
short circuit protected. The output of the switcher may
be set anywhere between 0.8V and 75% of Vin. Short
circuit protection is disabled during start-up to allow the
output capacitors time to fully charge.
Operating frequency of 200kHz, 600kHz or 1MHz
Input supply of 3V to 15V
0.5A Drive current for up to 10A output
Output voltages down to 0.8V
Overcurrent protection and soft start
SO-8 or MSOP-8 packages (C version offered in
MSOP package only)
Q3 R6
L1
R2
U2
SC2612
4
7
8
3
62
1
5
VCC
DH
BST
FB
DLSS/EN
COMP
GND
C2
R3
R1
R9
C7
C9
C3
R10
C1
1.5V OUT
Q2
12V IN
3.3V IN
C10
C5
Graphics IC Power supplies
Embedded, low cost, high efficiency converters
22003 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC2612A/B/C
Absolute Maximum Ratings
retemaraPlobmySmumixaMstinU
egatloVylppuStupnIV
CC
51V
egatloVniPtsooBV
BST
02V
DNGotLD
)2(
DNGotHD,
)2(
V
DLO
V,
DHI
02+ot1-V
egnaRerutarepmeTtneibmAgnitarepOT
A
07ot0C°
erutarepmeTnoitcnuJgnitarepOT
J
521C°
erutarepmeTegarotST
STG
051ot56-C°
s01)gniredloS(erutarepmeTdaeLT
LEAD
003C°
tneibmAotnoitcnuJecnatsiseRlamrehT
)3(
θ
JA
W/C°
esaCotnoitcnuJecnatsiseRlamrehT θ
JC
W/C°
)ledoMydoBnamuH(gnitaRDSEDSE2Vk
retemaraPlobmySsnoitidnoCniMpyTxaMstinU
egatloVylppuSCCVV
CC
0.351V
tnerruCtnecseiuQCCVI
QVCC
V
CC
V,V0.5=
BST
V0=NE/SS,V0.21=501Am
egatloVylppuSTSBV
BST
1102V
tnerruCtnecseiuQTSBI
QBST
V
CC
V,V0.5=
BST
V0=NE/SS,V0.21=5Am
tuokcoLegatloVrednUCCVVU
VCC
3.26.29.2V
tuokcoLegatloVrednUTSBVU
BST
0.70.80.9V
egatloVtuptuOV
OS
I
O
V;Am005=
FB
V=
OS
087008028Vm
egatlovpirttnerrucrevOV
ITS
4.07.0V
noitalugeRdaoLI
O
A4otA2.0=1%
noitalugeReniL 5.0±%
ycneuqerFrotallicsOf
OSC
A2162CS084006027
zHk
B2162CS00800010021
C2162CS061002042
elcyCytuDxaMrotallicsO δ
MAX
C2162CS,A2162CS08%
B2162CS07
egatloVnwodtuhSNE/SSV
SS
3.08.0V
tnerrucegrahCNE/SSI
SS
V8.0=ssV52µA
tnerruCecruoS/kniSHDkaeP,V5.4=HD-TSBV3.3=DNG-HD
V5.1=DNG-HD
5.0
05
A
Am
tnerruCecruoS/kniSLDkaeP,V5.4=LD-TSBV3.3=DNG-LD
V5.1=DNG-LD
5.0
05
A
Am
Electrical Characteristics
Unless specified: VCC = 3V to 12V; VFB = VO; BST = Vcc+5V; TA = 0 to 70°C
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.
3
2003 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC2612A/B/C
Notes:
(1) See Gate Resistor selection recommendations
(2) 1 square inch of FR4, double sided, 1oz. minimum copper weight.
Electrical Characteristics
Unless specified: VCC = 3V to 12V; VFB = VO; BST = Vcc+5V; TA = 0 to 70°C
retemaraPlobmySsnoitidnoCniMpyTxaMstinU
ecnatcudnocsnarTreifilpmArorrE
g
m8.0Sm
niaGreifilpmArorrEA
AE
R
PMOC
nepo=54Bd
tnerruCkniS/ecruoSreifilpmArorrE 06±Aµ
niaGrotaludoMA
M
V
CC
V5=91Bd
emiTdaeD 05sn
42003 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC2612A/B/C
Block Diagram
Pin Configuration Ordering Information
Pin Descriptions
Note:
(1) Only available in tape and reel packaging. A reel contains
2500 devices.
srebmuNtraP
)1(
ycneuqerFegakcaP
RTSMA2162CSzHk006
8-POSM
RTSMB2162CSzHM1
RTSMC2162CSzHk002
RTSA2162CSzHk006
8-OS
RTSB2162CSzHM1
BST
VCC
DH
GND
DLFB
SS/EN
1
2
3
4
TOP VIEW
(SO-8 or MSOP-8)
5
6
7
8
COMP
#niPemaNniPnoitcnuFniP
1PMOC.reifilpmarorreegatlovnoitcesrehctiwSehtfotuptuO
2NE/SS .etarpmaregatlovtuptuorehctiwsehtslortnoc,nipelbanednatratstfoS
3BF.tupnikcabdeeefnoitcesrehctiwS
4CCV.egatloVtupnIylppuSpihC
5DNG .senilediugtuoyalees,enalpdnuorgotyltceridtcennoc,dnuorGrewoPdnagolanA
6LD.tuptuoevirdTEFediswoLrehctiwS
7HD.tuptuoevirdTEFedishgiHrehctiwS
8TSB.sevirdTEFrofegatlovylppuS
DH
SS/EN
COMP
100mV
+
-
SYNCHRONOUS
MOSFET DRIVE
DL
R
S
Q
LEVEL SHIFT AND
HIGH SIDE DRIVE
+
-
UVLO
BST
OSCILLATOR
+
-
+
-
25uA
VREF
SSOVER
VCC
VREF
UVLO
&
REF
FB
SHDN
SHOOT-THRU
CONTROL
+
-
R
S
Q
GND
5
2003 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC2612A/B/C
Theory of Operation
The SC2612 is a step down DC/DC controller designed
for minimum cost and size without sacrificing accuracy
and protection. Overcurrent protection is implemented
by a simple undervoltage detection scheme and is dis-
abled until soft start has been completed to eliminate
false trips due to output capacitor charging. The SS/EN
pin is held low, as are the DH and DL pins, until the
undervoltage lockout points are exceeded. Once the VCC
and BST pins both rise above their undervoltage lockout
points, the SS capacitor begins to charge, controlling the
duty cycle of the switcher, and therefore slowly ramping
up the switcher output voltage. Once the SS capacitor is
charged, the current limit circuitry is enabled. If a short
circuit is applied , the output will be pulled down below
it’s trip point and shut down. The device may be restarted
by either cycling power, or momentarily pulling SS/EN low.
Component Selection
OUTPUT INDUCTOROUTPUT INDUCTOR
OUTPUT INDUCTOROUTPUT INDUCTOR
OUTPUT INDUCTOR - A good starting point for output
filter component selection is to choose an inductor value
that will give an inductor ripple current of approximately
20% of max. output current.
Inductor ripple current is given by:-
OSC
IN
O
O
RIPPLE
LfL
V
V
V
I⋅
−⋅
=
1
So choose inductor value from:-
OSCO
IN
O
O
fI
V
V
V
L⋅
−⋅⋅
=
15
OUTPUT CAPOUTPUT CAP
OUTPUT CAPOUTPUT CAP
OUTPUT CAPAA
AA
ACITCIT
CITCIT
CITOR(S) OR(S)
OR(S) OR(S)
OR(S) - The output capacitors should
be selected to meet output ripple and transient response
criteria. Output ripple voltage is caused by the inductor
ripple current flowing in the output capacitor’s ESR (There
is also a component due to the inductor ripple current
charging and discharging the output capacitor itself, but
this component is usually small and can often be ignored).
Given a maximum output voltage ripple requirement, ESR
is given by:-
OSC
IN
O
RIPPLEO
ESR fL
V
V
VV
R⋅
−⋅⋅
<
1
Output voltage transient excursions are a function of load
current transient levels, input and output voltages and
inductor and capacitor values.
Capacitance and RESR values to meet a required tran-
sient condition can be calculated from:-
release) (load transients positive for VV
and
n)applicatio (load transients negative for VVV
where
VV
IL
C
I
V
R
OA
OINA
AT
T
T
T
ESR
=
−=
⋅⋅ ⋅
>
<
2
2
values for positive and negative transients must be cal-
culated seperately and the worst case value chosen. For
Capacitor values, the calculated value should be doubled
to allow for duty cycle limitation and voltage drop issues.
62003 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC2612A/B/C
COMPENSACOMPENSA
COMPENSACOMPENSA
COMPENSATION COMPONENTSTION COMPONENTS
TION COMPONENTSTION COMPONENTS
TION COMPONENTS - Once the filter com-
ponents have been determined, the compensation com-
ponents can be calculated. The goal of compensation is
to modify the frequency response characteristics of the
error amplifier to ensure that the closed loop feedback
system has the highest gain and bandwidth possible while
maintaining stability.
A simplified stability criteria states that the open loop
gain of the converter should fall through 0dB at 20dB/
decade at a frequency no higher than 20-25% of the
switching frequency.
This objective is most simply met by generating asymp-
totic bode plots of the small signal response of the vari-
ous sections of the converter.
L
VOUT
Co
SC2612 AND FETS
FB
OUT
COMP Ra
MODULATOR
REF +
-
EA
Rb
Resr
Zp
Zf
Zs
It is convenient to split the converter into two sections,
the Error amp and compensation components being one
section and the Modulator, output filter and divider be-
ing the other.
First calculate the DC Filter+Modulator+Divider gain
The DC filter gain is always 0dB, the Modulator gain is
19dB at 5V in and is proportional to Vin, so modulator
gain at any input voltage is.
⋅+= 5
2019 IN
MOD
V
LogG
the divider gain is given by
+
⋅=
85
8
20
RR
R
LogGDIV
So the total Filter+Modulator+Divider DC Gain is
+
⋅+
⋅+=
BA
BIN
FMD RR
R
Log
V
LogG 20
5
2019
Calculate the filter double pole frequency (Fp(lc))
LCo
)lc(Fp π
=2
1
and calculate ESR Zero frequency (Fz(esr))
srReCo
)esr(Fz ⋅⋅π
=2
1
Choose an open loop crossover frequency (Fco) no higher
than 20% of the switching frequency (Fs).
The proximity of Fz(esr) to the crossover frequency Fco
determines the type of compensation required, if
Fz(esr)>Fco/4, use type 3 compensation, otherwise use
type 2. Type 1 compensation is not appropriate and is
not discussed here.
Type 2 Example
As an example of type 2 compensation, we will use the
Evaluation board schematic.
3.3uH
VOUT
3000uF
SC2612 AND FETS
FB
OUT
COMP 6.98k
MODULATOR
REF +
-
EA
8.06k
22mOhm
Cs
Cp
Rs
Vin=5V
The total Filter+Modulator+Divider DC Gain is
dB.
..
.
LogLogGFMD 613
068986
068
20
5
5
2019 =
+
⋅+
⋅+=
This is drawn as the line A-B in Fig2
kHz.
.
LCo
)lc(Fp 61
10300010332
1
2
1
66 ≈
⋅⋅⋅π
=
π
=−−
This is point B in Fig2.
kHz.)esr(Fz 42
10221030002
1
36 =
⋅⋅⋅⋅π
=−−
This is point C in Fig2., the line joining B-C slopes at -
40dB/decade, the line joining C-D slopes at -20dB/de-
cade.
For 600kHz switching frequency, crossover is designed
for 100kHz.
Since Fz(esr)<<Fco/4 Type 2 compensation is appropri-
ate.
7
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POWER MANAGEMENT
SC2612A/B/C
Having plotted the line ABCD, and confirmed the type of
compensation necessary, compensation component val-
ues can be determined.
At Fco, the line ABCD shows a gain of -27.5dB and a slope
of -20dB/decade. In order for the total open loop gain to
be 0dB with a -20dB/decade slope at this frequency, the
compensated error amp gain at Fco must be +27.5dB
with a 0dB slope. This is the line FG on the plot below.
Since open loop DC gain should be as high as possible to
minimize errors, a zero is placed at F and to minimize
high frequency gain and switching interference a pole is
placed at G.
The zero at F should be no higher than Fco/4 and the
pole at G no lower than 4*Fco. The equations to set the
gain and the pole and zero locations are:
dB) (in Fco at gain A where
gm
Rs
A
== 20
10
RsFz
Cs ⋅⋅π
=12
1
RsFp
Cp ⋅⋅π
=12
1
For this example, this results in the following values.
Ω≈Ω== kk.
.
Rs
.
30629
80
10 20
527
nF.Cs 220
103010256
1
33 =
⋅⋅⋅⋅
≈
) rolloffEA to duey (unecessar pFCp 14
1030104006
1
33 =
⋅⋅⋅⋅
≈
Fco
Fz(esr)
Fp(lc)
-60
-40
-20
0
20
40
60
80
100
100.0E+0 1.0E+3 10.0E+3 100.0E+3 1.0E+6
Frequency (Hz)
Gain (dB)
A B
C
D
E
FG
H
Fp1
Fz1
Filter+modulator
+divider gain
Compensated
Error Amp gain
Total open
loop gain
Fig2: Type 2 Error Amplifier Compensation
82003 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC2612A/B/C
Layout Guidelines
Careful attention to layout requirements are necessary
for successful implementation of the SC2612 PWM con-
troller. High currents switching at high frequency are
present in the application and their effect on ground plane
voltage differentials 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
bottom FET ground.
2). The loop formed by the Input Capacitor(s) (Cin), the
Top FET (Q1) and the Bottom FET (Q2) must be kept as
small as possible. This loop contains all the high current,
fast transition switching. Connections should be as wide
and as short as possible to minimize loop inductance.
Minimizing this loop area will a) reduce EMI, b) lower
ground injection currents, resulting in electrically “cleaner”
grounds for the rest of the system and c) minimize source
ringing, resulting in more reliable gate switching signals.
3). The connection between the junction of Q1, Q2 and
the output inductor should be a wide trace or copper
region. It should be as short as practical. Since this con-
nection has fast voltage transitions, keeping this con-
nection short will minimize EMI. The connection between
the output inductor and the output capacitors should be
a wide trace or copper area, there are no fast voltage or
current transitions in this connection and length is not
so important, however adding unnecessary impedance
will reduce efficiency.
L
12V IN
Cout
10uF
U1
SC2612
4
7
8
3
62
1
5
VCC
DH
BST
FB
DLSS/EN
COMP
GND
Q1
10
Vin
Vout
GND
0.1uF Cin
Q2
9
2003 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC2612A/B/C
Layout Guidelines (Cont.)
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) The SC2612 is best placed over a quiet ground plane
area, avoid pulse currents in the Cin, Q1, Q2 loop flowing
in this area. PGNDH and PGNDL should be returned to
the ground plane close to the package. The AGND pin
should be connected to the ground side of (one of) the
output capacitor(s). If this is not possible, the AGND pin
may be connected to the ground path between the Out-
put Capacitor(s) and the Cin, Q1, Q2 loop. Under no cir-
cumstances should AGND be returned to a ground in-
side the Cin, Q1, Q2 loop.
6) Vcc for the SC2612 should be supplied from the 5V
supply through a 10Ω resistor, the Vcc pin should be
decoupled directly to AGND by a 0.1µF ceramic capaci-
tor, trace lengths should be as short as possible.
Vout
Vin
+
+
Currents in Power Section
102003 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC2612A/B/C
Typical Characteristics
VIN = 5V
12V
70%
75%
80%
85%
90%
95%
100%
0246810
Output Current (A)
Efficiency (%)
VBST = 12V for VIN = 5V
VBST = 18V for VIN = 12V
Typical Efficiency
0%
20%
40%
60%
80%
100%
0.0 0.2 0.4 0.6 0.8 1.0 1.2
SS/EN Voltage (V)
Duty Cycle (%) (No Feedback)
SS/EN Control of duty cycle
1.490
1.492
1.494
1.496
1.498
1.500
4 5 6 7 8 9 10 11 12
VIN (V)
VO (V)
IO = 2.00A; VBST = 18V
Typical Line Regulation
5V
VIN = 12V
-2.0%
-1.5%
-1.0%
-0.5%
0.0%
0246810
IO (A)
VO (V)
VBST = 12V for VIN = 5V
VBST = 18V for VIN = 12V
Typical Load Regulation
11
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POWER MANAGEMENT
SC2612A/B/C
C9
0.1uF
GND
J8
C10
1500uF
C4
1500uF
Q2
Si4410DY
1.5V OUT
J4
C11
EMPTY
C2
10uF
L1 1uH
R3
2.2
GND
J5
Q1
Si4410DY
R1
10
12V IN
J1
R2
2.2
GND
J6 U1
SC2612
4
7
8
3
62
1
5
VCC
DH
BST
FB
DLSS/EN
COMP
GND
EN
J11
C1 0.1uF
D1
EMPTY
C8
220pF
R7
30k
R5
6.98k
R8
8.06k
3.3V - 12V IN
J2
C6
1500uF
GND
J7
C5
1500uF
C3
1500uF
Evaluation Board Schematic & Layout
122003 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC2612A/B/C
JEDEC REF: MS-012AA
Land Pattern - SO-8
Outline Drawing - SO-8
13
2003 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC2612A/B/C
Outline Drawing - MSOP-8
Land Pattern - MSOP-8
Contact Information
Semtech Corporation
Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805)498-2111 FAX (805)498-3804