Dual Synchronous Buck Pseudo Fixed
Frequency DDR Power Supply Controller
POWER MANAGEMENT
Description
The SC1486 is a dual output constant on synchronous-
buck PWM controller optimized for cost effective mobile
DDR applications. Features include high efficiency, a fast
dynamic response with no minimum on time, a REFIN
input and a buffered REFOUT pin capable of sourcing
3mA. The excellent transient response means that
SC1486 based solutions will require less output
capacitance than competing fixed frequency converters.
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.
The output voltage of the first controller can be adjusted
from 0.5V to VCCA. In DDR applications, this voltage is
set to 2.5 volts. A resistor divider from the 2.5 volt supply
is used to drive the REFIN pin of the second controller. A
unity gain buffer drives the REFOUT pin to the same
potential as REFIN. The second controller regulates its
output to REFOUT. Two frequency setting resistors set
the on-time for each buck controller. The frequency can
thus be tailored to minimize crosstalk. The integrated
gate drivers feature adaptive shoot-through protection
and soft switching. Additional features include cycle-by-
cycle current limit, digital soft-start, overvoltage and
under-voltage protection, and a PGOOD output for each
controller.
SC1486
Features
Constant on-time for fast dynamic response
VIN range = 1.8V – 25V
DC current sense using low-side RDS(ON) sensing
or sense resistor
Integrated reference buffer for VTT
Low power S3 state
Resistor programmable frequency
Cycle-by-cycle current limit
Digital soft-start
PSAVE option for VDDQ
Over-voltage/under-voltage fault protection
<20uA shutdown current
Low quiescent power dissipation
Two separate PGOOD indicators
Separate enable of each switcher
Integrated gate drivers with soft switching
Efficiency >90%
1% Internal reference
28 Lead TSSOP
Industrial temperature range
Applications
Notebook computers
CPU I/O supplies
Handheld terminals and PDAs
LCD monitors
Network power supplies
Typical Application Circuit
R1
VBAT
23
R2
9
VDDQ/2
8
10
22
TON2
REFIN
TON1
PGOOD1
27
13
PGOOD2
VBAT
PGOOD1
SC1486
PGOOD2
Q4
REFOUT
REFOUT
EN/PSV1
DH2
LX2
ILIM2
Q1
6
5
R3
4
R4
R11
1
PGND1
R10
R5
PGND1
+
C7
Q2
2
ILIM1
DL1
FBK2
FBK1
12
26
PGND1
DH1
LX1
DL2
PGND2
20
19
R8
18
16
15
Q3
+
C8
EN1
L2
VTT, 1.25V
VDDQ, 2.5V
L1
Revision 5, January 2003
1
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