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PDF SC470 Data sheet ( Hoja de datos )
| Número de pieza | SC470 | |
| Descripción | Synchronous Buck Controller | |
| Fabricantes | Semtech Corporation | |
| Logotipo |  |
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SC470
Synchronous Buck Controller for
Dynamic Load-Voltage Applications
POWER MANAGEMENT
Description
Features
The SC470 is a single output, constant on-time
synchronous-buck, pseudo-fixed frequency, PWM
controller intended for use in notebook computers and
other battery operated portable devices. Features
include high efficiency and fast dynamic response with
no minimum on-time. The excellent transient response
means that SC470 based solutions will require less
output capacitance than competing fixed frequency
converters.
www.DataSheet4U.com
The SC470 is specifically targeted for graphics processor
power supplies that require dynamic voltage transition,
with a tight 0.85% DC accuracy and a 20% OVP threshold.
The 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 or input voltage. After the transient event, the
controller frequency will return to steady state operation.
At light loads, Power-Save Mode enables the SC470 to
skip PWM pulses for better efficiency.
The output voltage can be adjusted from 0.5V to VCCA.
The integrated gate drivers feature adaptive shoot-
through protection and soft switching. Additional features
include cycle-by-cycle current limit, digital soft-start, over-
voltage and under-voltage protection, and a PGD output.
Typical Application Circuit
Constant on-time for fast dynamic response
Programmable VOUT range = 0.5 – VCCA
VBAT range = 1.8V – 25V
DC current sense using low-side RDS(ON) sensing
or RSENSE in source of low-side MOSFET for
greater accuracy
Resistor programmable on-time
Cycle-by-cycle current limit
Digital soft-start
Combined EN and PSAVE functions
Over-voltage/under-voltage fault protection
and PGD output
20% OVP threshold for simpler dynamic voltage
transition circuitry
5µA typical shutdown current
Low quiescent power dissipation
14 lead TSSOP and 16 pin MLPQ (4mm x 4mm)
packages
Industrial temperature range
0.85% DC accuracy
Integrated gate drivers with soft-switching
Applications
Graphics Cards
Embedded Graphics Processors
High Performance Processors
VBAT
5VSUS
5VSUS
VBAT
PGOOD
R1
RTON
R1
10R
VOUT
R2
R2
R4
C5 C6
1nF 1uF
U1
EN/PSV
TON
VOUT
VCCA
FB
PGD
VSSA
SC470
BST
DH
LX
ILIM
VDDP
DL
PGND
D1
C1 0.1uF
R3
C4
1uF
Q1 C2
10uF
L1
Q2
C3
+
VOUT
September 27, 2005
1
www.semtech.com
1 page  
POWER MANAGEMENT
Pin Configuration
16 15 14 13
VOUT
VCCA
www.DataSheet4U.com
FB
PGD
1
2
3
4
TOP VIEW
T
12 DH
11 LX
10 ILIM
9 VDDP
5678
SC470
Ordering Information
DEVICE(1)
PACKAGE
SC470IMLTRT(2)(3)
MLPQ-16
SC470ITSTRT(2)(3)
TSSOP-14
SC470EVB(4)
Evaluation Board
Notes:
(1) Only available in tape and reel packaging. A reel contains
2500 devices.
(2) This product is fully WEEE and RoHS compliant.
(3) Lead-free product. This product is J-STD-020B compliant and
all homogeneous subcomponents are RoHS compliant.
(4) Part-specific evaluation boards - consult factory for availability.
MLPQ16: 4X4 BODY
EN/PSV
T ON
VOUT
VCCA
FB
PGD
VSSA
TOP VIEW
1 14
2 13
3 12
4 11
5 10
69
78
BST
DH
LX
ILIM
VDDP
DL
PGND
(14 Pin TSSOP)
2005 Semtech Corp.
5
www.semtech.com
5 Page 
SC470
POWER MANAGEMENT
Application Information (Cont.)
(maximum) minimum off-time one-shot. For best dropout
performance, use the slowest on-time setting of 200kHz.
When working with low input voltages, the duty-factor
limit must be calculated using worst-case values for on
and off times. The IC duty-factor limitation is given by:
DUTY =
t ON(MIN)
+t ON(MIN) t OFF(MAX )
Design Procedure
Prior to designing an output and making component
selections, it is necessary to determine the input voltage
range and the output voltage specifications. For purposes
of demonstrating the procedure the output for the
schematic in Figure 8 on Page 17 will be designed.
www.DataBSheeestu4Ure.cotmo include inductor resistance and MOSFET on-
state voltage drops when performing worst-case dropout
duty-factor calculations.
470 System DC Accuracy
Two IC parameters affect system DC accuracy, the error
comparator threshold voltage variation and the switching
frequency variation with line and load. The error
comparator threshold does not drift significantly with
supply and temperature. Thus, the error comparator
contributes 0.85% or less to DC system inaccuracy.
Board components and layout also influence DC
accuracy. The use of 1% feedback resistors contribute
1%. If tighter DC accuracy is required use 0.1% feedback
resistors.
The on-pulse in the SC470 is calculated to give a pseudo-
fixed frequency. Nevertheless, some frequency variation
with line and load can be expected. This variation changes
the output ripple voltage. Because constant on-
regulators regulate to the valley of the output ripple, ½
of the output ripple appears as a DC regulation error.
For example, if the feedback resistors are chosen to
divide down the output by a factor of five, the valley of
the output ripple will be VOUT. For example: if VOUT is
2.5V and the ripple is 50mV with VBAT = 6V, then the
measured DC output will be 2.525V. If the ripple increases
to 80mV with VBAT = 25V, then the measured DC output
will be 2.540V.
The maximum input voltage (VBAT(MAX)) is determined by
the highest AC adaptor voltage. The minimum input
voltage (VBAT(MIN)) is determined by the lowest battery
voltage after accounting for voltage drops due to
connectors, fuses and battery selector switches. For the
purposes
20V.
of
this
design
we
will
use
a
VBAT
range
of
8V
to
Four parameters are needed for the output:
1) Nominal output voltage, VOUT (we will use 1.2V).
2) Static (or DC) tolerance, TOLST (we will use +/-4%).
3) Transient tolerance, TOLTR and size of transient (we
will use +/-8% for purposes of this demonstration).
4) Maximum output current, IOUT (we will design for 6A).
Switching frequency determines the trade-off between
size and efficiency. Increased frequency increases the
switching losses in the MOSFETs, since losses are a
function of VIN2, knowing the maximum input voltage and
budget for MOSFET switches usually dictates where the
adsesaigsntaerntdinsguppo. inAtd, ebfuatutlht iRstOiNs
value of
not set
1MΩ is suggested
in stone. The first
thing to do is to calculate the on-time, tON, at VBAT(MIN) and
V ,BAT(MAX) since this depends only upon VBAT, VOUT and RtON.
For VOUT < 3.3V:
( )tON_ VBAT(MIN) = 3.3 • 10−12 • RtON + 37 • 103
•
VOUT
VBAT(MIN)
+
50
• 10−9
s
The output inductor value may change with current. This
will change the output ripple and thus the DC output
voltage. It will not change the frequency.
Switching frequency variation with load can be minimized
by choosing MOSFETs with lower RDS(ON). High RDS(ON)
MOSFETs will cause the switching frequency to increase
as the load current increases. This will reduce the ripple
and thus the DC output voltage.
and,
( )tON_ VBAT(MAX) = 3.3 • 10−12 •
RtON + 37 • 103
•
VOUT
VBAT ( MAX )
+
50
• 10−9 s
From these values of tON we can calculate the nominal
switching frequency as follows:
2005 Semtech Corp.
11
www.semtech.com
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet SC470.PDF ] | |
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