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PDF SC412A Data sheet ( Hoja de datos )
| Número de pieza | SC412A | |
| Descripción | Synchronous Buck Controller | |
| Fabricantes | Semtech Corporation | |
| Logotipo |  |
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SC412A
Synchronous Buck Controller
POWER MANAGEMENT
Description
The SC412A is a versatile, constant on-time synchronous
buck, pseudo fixed-frequency, PWM controller intended
for notebook computers and other battery operated
portable devices. The SC412A contains all the features
needed to provide cost-effective control of switch-mode
power outputs.
The output voltage is programmable from 0.75 to 5.25
www.DatavSohletest4Uu.scionmg external resistors. Switching frequency is
internally preset to 325kHz. Additional features cycle-
by-cycle current limit, voltage soft-start, under-voltage
protection, programmable over-current protection, soft
shutdown, automatic power save and non-overlapping
gate drive. The SC412A also provides an enable input
and a power good output.
The constant on-time topology provides fast, dynamic
response. The excellent transient response means that
SC412A based solutions require less output capacitance
than competing fixed-frequency converters. Switching
frequency is constant until a step in load or line voltage
occurs, at which time the pulse density and frequency
will increase or decrease to counter the change in
output voltage. After the transient event, the controller
frequency returns to steady state operation. At light loads,
the automatic power save mode reduces the switching
frequency for improved efficiency.
Features
VBAT Range 3V to 25V
Soft-Shutoff at Output
Current Sense Using Low-Side RDS(ON) or Resistor
Sensing with Adjustable Cycle-by-Cycle Current Limit
Fixed-Frequency 325kHz
Constant On-Time for Fast Dynamic Response and
Reduced Output Capacitance
Automatic Smart Power Save†
Internal Soft-Start
Over-Voltage/Under-Voltage Fault Protection
Power Good Output
1μA Typical Shutdown Current
Tiny 3x3mm, 16 Pin MLP, Lead-free Package
Low External Part Count
Industrial Temperature Range
1% Internal Reference
1A/3A Non-Overlapping Gate Drive with SmartDrive™
High Efficiency > 90%
Fully WEEE and RoHS Compliant
†Patent pending
Applications
Notebook and Sub-Notebook Voltage Controllers
Tablet PCs
Embedded Applications
Typical Application Circuit
VCC
VBAT
VOUT
C1
D1
CIN Q1
C2
L1
COUT
Q2 VCC
VCC
RLIM
R1
1 LX
EN 12
2 BST
U1 PGOOD 11
3
4
VCC
DL
SC412A VOUT
FB
10
9
EN
PGOOD
VOUT
FB
R2
September 19, 2006
R3
1
www.semtech.com
1 page  
POWER MANAGEMENT
Pin Configuration
16 15 14 13
LX
BST
1
2
TOP VIEW
12 EN
11 PGOOD
www.DataSheet4VUC.cCom
DL
3
4
10 VOUT
T 9 FB
5678
SC412A
Ordering Information
Device
SC412AMLTRT(1)
SC412AEVB
Package(2)
MLPQ-16 3X3
Evaluation Board
Notes:
1) Available in tape and reel packaging only. A reel contains 3000
devices.
2) Available in lead-free packaging only. This product is fully WEEE,
RoHS and J-TD-020B compliant. This component and all homog-
enous sub-components are RoHS compliant.
MLPQ16: 3X3 16 LEAD
Marking Information
412A
yyww
xxxx
Marking for the 3 x 3mm MLPQ 16 Lead Package
nnnn = Part Number (example: 412A)
yyww = Date Code (example: 0652)
xxxx = Semtech Lot No. (example: E901)
© 2006 Semtech Corp.
5
www.semtech.com
5 Page 
SC412A
POWER MANAGEMENT
Applications Information (continued)
Soft-Start
The soft-start is accomplished by ramping the FB com-
parator’s internal reference from zero to 0.75V in 30mV
increments. Each 30mV step typically lasts for eight clock
cycles.
During the soft-start period, the Zero Cross Detector is
active to monitor the voltage across the lower MOSFET
while DL is high. If the inductor current reaches zero, the
www.DataFSBheecto4Um.cpoamrator’s internal ramp reference is immediately
overridden to match the voltage at the FB pin. This soon
causes the FB comparator to trip which forces DL to turn
off and a DH on-time will begin. This prevents the inductor
current from going too negative which would cause droop
in the VOUT start-up waveform. The next 30mV step on the
internal reference ramp occurs from the new point at the
FB pin. Since any of the internal 30mV steps can be over-
ridden by the FB waveform, the start-up time is therefore
dependent upon operating conditions. This override feature
will stop when the FB pin reaches approximately 660mV.
At start-up, during the first 32 switching cycles, the over-
current threshold is reduced by 50%, to reduce overshoot
caused by the first set of switching pulses.
MOSFET Gate Drivers
The DH and DL drivers are optimized for driving moderate
high-side and larger low-side power MOSFETs. An adaptive
dead-time circuit monitors the DL output and prevents the
high-side MOSFET from turning on until DL is fully off, and
conversely, monitors the DH output and prevents the low-
side MOSFET from turning on until DH is fully off. Be sure
there is low resistance and low inductance between the DH
and DL outputs to the gate of each MOSFET.
The SC412A utilizes SmartDriveTM to achieve fast switching
with reduced noise. At the start of the DH on-time when
LX is typically below GND, the DH output drives the high-
side MOSFET through a pull-up resistance of 10 ohms,
which results in a soft reverse-recovery of the low-side
diode. The high-side MOSFET conducts and causes LX to
rise; when LX reaches 1.5volts, the DH drive resistance is
reduced to 2 ohms to provide fast switching and reduce
switching loss.
Design Procedure
Prior to designing a switch mode supply, the input voltage,
load current, switching frequency and inductor ripple cur-
rent must be specified.
For notebook systems the maximum input voltage (VINMAX)
is determined by the highest AC adaptor voltage, and the
minimum input voltage (VINMIN) is determined by the lowest
battery voltage after accounting for voltage drops due to
connectors, fuses and battery selector switches.
In general, four parameters are needed to define the
design:
1) Nominal output voltage (VOUT)
2) Static or DC output tolerance
3) Transient response
4) Maximum load current (IOUT)
There are two values of load current to consider: continu-
ous load current and peak load current. Continuous load
current is concerned with thermal stresses which drive
the selection of input capacitors, MOSFETs and commuta-
tion diodes. Peak load current determines instantaneous
component stresses and filtering requirements such as
inductor saturation, output capacitors and design of the
current limit circuit.
Design example:
VBAT = 10V min, 20V max
VOUT = 1.15V +/- 4%
Load = 20A maximum
Inductor Selection
Low inductor values result in smaller size, but create high-
er ripple current and are less efficient because of the high
AC current flowing in the inductor. Higher inductor values
will reduce the ripple current and are more efficient, but
are larger and more costly. The inductor selection is gen-
erally based on the ripple current which is typically set
between 20% to 50% of the maximum load current. Cost,
size, output ripple and efficiency all play a part in the se-
lection process.
© 2006 Semtech Corp.
11
www.semtech.com
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| Páginas | Total 22 Páginas | |
| PDF Descargar | [ Datasheet SC412A.PDF ] | |
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