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PDF SC4608 Data sheet ( Hoja de datos )
| Número de pieza | SC4608 | |
| Descripción | High Efficiency Synchronous Buck | |
| Fabricantes | Semtech Corporation | |
| Logotipo |  |
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POWER MANAGEMENT
Description
The SC4608 is a voltage mode step down (buck) regula-
tor controller that provides accurate high efficiency power
conversion from an input supply range of 2.7V to 5.5V. A
high level of integration reduces external component
count, and makes it suitable for low voltage applications
where cost, size and efficiency are critical. The SC4608
is capable of producing an output voltage as low as 0.5V.
It’s frequency of operation is programmable to 1MHz.
www.DataTShheeetS4UC.4co6m08 drives external, N-channel MOSFETs with a
peak gate current of 1A. A non-overlap protection is pro-
vided for the gate drive signals to prevent shoot through
of the MOSFET pair. The SC4608 features lossless cur-
rent sensing of the voltage drop across the drain to
source resistance of the high side MOSFET during its
conduction period.
The quiescent supply current in sleep mode is typically
lower than 1µA. A external soft start is provided to pre-
vent output voltage overshoot during start-up.
The SC4608 is an ideal choice for converting 3.3V, 5V or
other low input supply voltages. It’s available in 16 pin
MLP package.
Typical Application Circuit
SC4608
Low Input, MHz Operation,
High Efficiency Synchronous Buck
Features
Asynchronous start up
Programmable switching frequency up to 1MHz
BiCMOS voltage mode PWM controller
2.7V to 5.5V input voltage range
Output voltage as low as 0.5V
+/-1% reference accuracy
Sleep mode (Icc = 1µA max)
Adjustable lossless short circuit current limiting
Combination pulse by pulse & hiccup mode
current limit
High efficiency synchronous switching
1A peak current driver
External soft start
Power good signal
16-pin MLP Lead-free package. This product is fully
WEEE and RoHS compliant
Applications
Distributed power architecture
Servers/workstations
Local microprocessor core power supplies
DSP and I/O power supplies
Battery-powered applications
Telecommunications equipment
Data processing applications
C1
180p
R13
R3 1
C3
1u
C2 2.2n
C16
560pF
R1
14.3k
16
1
2
3
4
R2
10k
Css
22n
5
6
7
SC4608
PVDD
BST
AVDD
DRVH
ISET
COMP
PHASE
FSET
DRVL
EN PGND
PGOOD
AGND
SS VSENSE
15
13
12
11
10
9
8
R6
1
R5
1
D2
M11
C17
1u
M2
C10
220u
Vin=3V – 3.6V
C13 C14
22u 22u
L1
1.8u
C6
330u
C5
22u
Vout= 0.5V / 12A
C4 C9
22u 4.7n
R7
10k
R8
200
Revision: June 19, 2006
1
www.semtech.com
1 page  
POWER MANAGEMENT
Pin Configuration
TOP VIEW
www.DataSheet4U.com
SC4608
Ordering Information
Part Number
Device(1)
SC4608MLTRT(2)
MLP-16
SC4608EVB
Evaluation Board
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.
(16 Pin MLP)
Pin Descriptions
Pin #
1
2
3
4
5
Pin Name
AVDD
ISET
COMP
FSET
EN
Pin Function
Power supply voltage for the analog section of the controller.
The ISET pin is used to limit current in the high side MOSFET. The SC4608 uses the
voltage across the VIN and ISET pins in order to set the current limit. The current limit
threshold is set by the value of an external resistor (R3 in the Typical Application Circuit
Diagram). Current limiting is performed by comparing the voltage drop across the sense
resistor with the voltage drop across the drain to source resistance of the high side
MOSFET during the MOSFET’s conduction period. The voltage drop across the drain to
source resistance of the high side MOSFET is obtained from the VIN and PHASE pin.
This is the output of the voltage error amplifier. The voltage at this output is inverted
internally and connected to the non-inverting input of the PWM comparator. A lead-lag
network from the COMP pin to the VSENSE pin compensates for the two pole LC filter
characteristics inherent to voltage mode control. The lead-lag network is required in order
to optimize the dynamic performance of the voltage mode control loop.
The FSET pin is used to sets the PWM oscillator frequency through an external timing
capacitor that is connected from the FSET pin to the GND pin. The SC4608 can be
operated in synchronous mode by placing a resistor in series between the timing capacitor
and ground. The other terminal of the timing capacitor will remain connected to the FSET
pin.
The oscillator frequency of the SC4608 is set by FSET when EN is pulled and held above
0.7 * AVDD. Its shutdown mode is invoked if EN is pulled and held below 0.3 * AVDD.
2006 Semtech Corp.
5
www.semtech.com
5 Page 
SC4608
POWER MANAGEMENT
Application Information (Cont.)
mode, the RMS value of the input capacitor can be cal-
culated from:
ICIN(RMS) = IOMAX ⋅
VOUT ⋅ (VIN − VOUT )
V
2
IN
This current gives the capacitor’s power loss as follows:
PCIN
=
I2
CIN( RMS )
⋅ RCIN(ESR)
Where:
IB = the boost current and
VD= discharge ripple voltage.
With fs = 300kH, VD=0.3V and IB = 50mA, the required
capacitance for the boost capacitor is:
Cboost
=
IB
VD
⋅
1
fs
⋅ Dmax
=
0.05
0.3
1
⋅ 300k
⋅ 0.95
= 528nF
www.DataTShheisetc4Ua.pcoamcitor’s RMS loss can be a significant part of the
total loss in the converter and reduce the overall con-
verter efficiency. The input ripple voltage mainly depends
on the input capacitor’s ESR and its capacitance for a
given load, input voltage and output voltage. Assuming
that the input current of the converter is constant, the
required input capacitance for a given voltage ripple can
be calculated by:
CIN
= IOMAX
⋅
fs ⋅ (∆VI
D ⋅ (1− D)
− IOMAX ⋅ RCIN(ESR) )
Where:
D = VO/VI , duty ratio and
∆VI = the given input voltage ripple.
Because the input capacitor is exposed to the large surge
current, attention is needed for the input capacitor. If
tantalum capacitors are used at the input side of the
converter, one needs to ensure that the RMS and surge
ratings are not exceeded. For generic tantalum capaci-
tors, it is wise to derate their voltage ratings at a ratio of
2 to protect these input capacitors.
Boost Capacitor Selection
The boost capacitor selection is based on its discharge
ripple voltage, worst case conduction time and boost
current. The worst case conduction time Tw can be esti-
mated as follows:
Tw
=
1
fs
⋅ Dmax
Where:
fs = the switching frequency and
Dmax = maximum duty ratio.
The required minimum capacitance for boost capacitor
will be:
Cboost
=
IB
VD
⋅ TW
Power MOSFET Selection
The SC4608 can drive an N-MOSFET at the high side
and an N-MOSFET synchronous rectifier at the low side.
The use of the high side N-MOSFET will significantly re-
duce its conduction loss for high current. For the top
MOSFET, its total power loss includes its conduction loss,
switching loss, gate charge loss, output capacitance loss
and the loss related to the reverse recovery of the bot-
tom diode, shown as follows:
PTOP _ TOTAL
=
I2
TOP _ RMS
⋅ RTOP _ ON
+
ITOP _ PEAK
VGATE
⋅ VI
RG
⋅ fs
⋅
(QGD + QGS2 ) + QGT ⋅ VGATE ⋅ fs + (QOSS + Qrr ) ⋅ VI ⋅ fs
Where:
RG = gate drive resistor,
QGD = the gate to drain charge of the top MOSFET,
QGS2 = the gate to source charge of the top MOSFET,
QGT = the total gate charge of the top MOSFET,
QOSS = the output charge of the top MOSFET and
Qrr = the reverse recovery charge of the bottom diode.
For the top MOSFET, it experiences high current and high
voltage overlap during each on/off transition. But for the
bottom MOSFET, its switching voltage is the bottom
diode’s forward drop during its on/off transition. So the
switching loss for the bottom MOSFET is negligible. Its
total power loss can be determined by:
PBOT _ TOTAL
=
I2
BOT _ RMS
⋅ RBOT _ ON
+ QGB ⋅ VGATE
⋅ fs
+ ID _ AVG ⋅ VF
Where:
QGB = the total gate charge of the bottom MOSFET and
VF = the forward voltage drop of the bottom diode.
2006 Semtech Corp.
11
www.semtech.com
11 Page | ||
| Páginas | Total 19 Páginas | |
| PDF Descargar | [ Datasheet SC4608.PDF ] | |
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