500kHz Step-Down DC/DC Converter
POWER MANAGEMENT
Description
The SC2612E is a voltage mode switcher designed for
low cost, “point of use” voltage conversion. SC2612E is
available with fixed switching frequencies of 500kHz.
The SC2612E 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.
SC2612E
Features
u
u
u
u
u
u
Operating frequency of 500kHz
Input supply of 4.5V to 15V
0.5A Drive current for up to 10A output
Output voltages down to 0.8V
Overcurrent protection and soft start
SO-8 package
Applications
u
Graphics IC Power supplies
u
Embedded, low cost, high efficiency converters
Typical Application Circuit
12V IN
5V IN
R1
C10
C1
C2
U2
2
7
8
C5
C7
C9
R9
R10
4
VCC
COMP
SS/EN
GND
BST
DH
DL
FB
6
5
3
1
Q3
C3
R6
R2
R3
Q2
L1
1.5V OUT
SC2612E
Revision: October 12, 2004
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SC2612E
POWER MANAGEMENT
Absolute Maximum Ratings
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.
Parameter
VCC Supply Voltage
Boost Pin Voltage
DL to GND
(1)
, DH to GND
(1)
DH to GND Negative Pulse (t
pulse
< 10ns)
DL to GND Negative Pulse (t
pulse
< 20ns)
Operating Ambient Temperature Range
Operating Junction Temperature
Storage Temperature
Lead Temperature (Soldering) 10s
Thermal Resistance Junction to Ambient
Thermal Resistance Junction to Case
ESD Rating (Human Body Model)
(2)
Symbol
V
CC
V
BST
V
DLO
, V
DHI
V
DH_PULSE
V
DL_PULSE
T
A
T
J
T
STG
T
LEAD
θ
JA
θ
JC
ESD
Maximum
18
20
-1 to +20
-4.5
-4.5
0 to 70
125
-65 to 150
300
113
42
2
Units
V
V
V
V
V
°C
°C
°C
°C
°C/W
°C/W
kV
Electrical Characteristics
Unless specified: V
CC
= 4.5V to 12V; V
FB
= V
O
; BST = Vcc+5V; T
A
= 0 to 70°C
Parameter
VCC Supply Voltage
VCC Quiescent Current
BST Supply Voltage
BST Quiescent Current
VCC Under Voltage Lockout
BST Under Voltage Lockout
Output Voltage
Overcurrent trip voltage
Load Regulation
Line Regulation
Oscillator Frequency
Oscillator Max Duty Cycle
SS/EN Shutdown Voltage
SS/EN Charge current
Peak DH Sink/Source Current
Peak DL Sink/Source Current
Symbol
V
CC
I
QVCC
V
BST
I
QBST
UV
VCC
UV
BST
V
OS
V
ITS
Conditions
Min
4.5
Typ
Max
15
Units
V
mA
V
mA
V
V
mV
V
%
%
V
CC
= 5.0V, V
BST
= 12.0V, SS/EN = 0V
4.5
V
CC
= 5.0V, V
BST
= 12.0V, SS/EN = 0V
3.8
3.15
I
O
= 10mA; V
FB
= V
OS
, T
A
= 25°C
I
O
= 0.2A to 4A
792
0.4
5
10
18
5
4.15
3.5
800
4.5
3.85
808
0.7
1
±0.5
f
OSC
δ
MAX
V
SS
I
SS
Vss = 0.8V
BST - DH = 4.5V,
BST - DL = 4.5V,
DH - GND = 3.3V
DH - GND = 1.5V
DL - GND = 3.3V
DL - GND = 1.5V
400
80
0.3
500
600
kHz
%
0.8
25
V
µA
A
mA
A
mA
0.5
50
0.5
50
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SC2612E
POWER MANAGEMENT
Electrical Characteristics
Unless specified: V
CC
= 4.5V to 12V; V
FB
= V
O
; BST = Vcc+5V; T
A
= 0 to 70°C
Parameter
Error Amplifier Transconductance
Error Amplifier Gain
(3)
Error Amplifier Source/Sink Current
Modulator Gain
(3)
Dead Time
(3)
Symbol
Conditions
Min
Typ
0.8
Max
Units
mS
dB
µA
dB
ns
g
m
A
EA
A
M
R
COMP
= open
V
C C
= 5V
45
± 60
19
50
Notes:
(1) See Gate Resistor selection recommendations.
(2) 1square inch of FR4, double sided, 1oz. minimum copper weight.
(3) Guaranteed by design, not tested in production.
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SC2612E
POWER MANAGEMENT
Pin Configuration
TOP VIEW
FB
VCC
DL
GND
1
2
3
4
8
7
6
5
SS/EN
COMP
BST
DH
Ordering Information
Part Numbers
(1)
SC2612ESTRT
(2)
Frequency
500kHz
P ackag e
SO-8
Note:
(1) Only available in tape and reel packaging. A reel contains
2500 devices.
(2) Lead free product. This product is fully WEEE and RoHS
compliant.
(SO-8)
Pin Descriptions
Pin #
1
2
3
4
5
6
7
8
Pin Name
FB
VC C
DL
GND
DH
BST
COMP
SS/EN
Switcher section feeedback input.
Chip Supply Input Voltage.
Switcher Low side FET drive output.
Analog and Power Ground, connect directly to ground plane, see layout guidelines.
Switcher High side FET drive output.
Supply voltage for FET drives.
Output of the Switcher section voltage error amplifier.
Soft start and enable pin, controls the switcher output voltage ramp rate.
Pin Function
Block Diagram
VCC
VREF
UVLO
+
-
UVLO
&
REF
LEVEL SHIFT AND
HIGH SIDE DRIVE
BST
DH
SHDN
+
R
Q
SHOOT -T HRU
CONT RO
L
FB
COMP
SS/EN
-
-
+
S
VREF
25uA
OSCILLAT OR
-
+
+
-
SSOVER
R
Q
S
SYNCHRONOUS
MOSFET DRIVE
DL
GND
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SC2612E
POWER MANAGEMENT
Theory of Operation
The SC2612E 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 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:-
æ
V
ö
V
O
× ç
1
-
O
÷
ç
V
÷
IN
ø
è
=
L
×
f
OSC
I
L RIPPLE
So choose inductor value from:-
æ
V
ö
5
×
V
O
× ç
1
-
O
÷
ç
V
÷
IN
ø
è
L
=
I
O
×
f
OSC
OUTPUT CAPACITOR(S) - The output capacitors should
CAPACITOR(S)
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:-
æ
V
ö
V
O
×
V
RIPPLE
× ç
1
-
O
÷
ç
V
÷
IN
ø
è
<
L
×
f
OSC
R
ESR
Output voltage transient excursions are a function of load
current transient levels, input and output voltages and
inductor and capacitor values.
Capacitance and R
ESR
values to meet a required tran-
sient condition can be calculated from:-
R
ESR
<
C
>
V
T
I
T
2
L
×
I
T
2
×
V
T
×
V
A
where
V
A
=
V
IN
-
V
O
for negative transients (load application)
and
V
A
=
V
O
for positive transients (load release)
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.
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