sc1175 Semtech Corporation, sc1175 Datasheet - Page 7

• Manufacturer Part Number: sc1175
• Description: Sc1175 Low-power Dual Synchronous Dc/dc Controller With Current-sharing Circuitry
• Manufacturer: Semtech Corporation

POWER MANAGEMENT
Power Good
The controller provides a power good signal. This is an
open collector output, which is pulled low if the output
voltage is outside of the power good window.
Soft Start/Enable
The Soft Start/Enable (SS/ENA) pin serves several func-
tions. If held below the Soft Start Enable threshold, both
channels are inhibited. DH1 and DH2 will be low, turning
off the top FETs. Between the Soft Start Enable thresh-
old and the Soft Start End threshold, the duty cycle is
allowed to increase. At the Soft Start End threshold,
maximum duty cycle is reached. In practical applications
the error amplifier will be controlling the duty cycle be-
fore the Soft Start End threshold is reached. To avoid
boost problems during startup in current share mode,
both channels start up in asynchronous mode, and the
bottom FET body diode is used for recirculating current
during the FET off time. When the SS/ENA pin reaches
the Soft Start Transition threshold, the channels begin
operating in synchronous mode for improved efficiency.
The soft start pin sources approximately 25uA and soft
start timing can be set by selection of an appropriate
soft start capacitor value.
SENSE RESISTOR SELECTION
Current Sharing Mode
Calculation of the three programming resistors to achieve
sharing. Three resistors will determine the current shar-
ing load line. First the offset resistor will ensure that the
load line crosses the origin (0 Amp on each channel) for
sharing at light current. A pull up resistor from the 5V
bias (V
the slave channel (below 50%), the pull up will be on pin
Applications Information - Theory of Operation
2004 Semtech Corp.
their own current limits, identical to the independent
channel case.
CC
of the chip) will be used. For low duty cycle on
7
3. For high duty cycle on the slave channel (above 50%),
the pull up will be on pin 2.
The formula is:
100
pins 2 and 3 to the two output sense resistors.
.1 V is an estimated voltage drop across the MOSFETs.
Positive values go to pin 3, negative to pin 2.
A +20K will be a 20K on pin 3.
A -20K will be a 20K on pin 2.
Now that the offset resistor has been fixed, we need to
set up the maximum current for each channel.
Selection of R
R
Selection of R
R
The errors will be minimized if the power components
have been sized proportionately to the maximum
currents.
Independent Channels
Calculation of the two current limiting resistors.
There is no need for an offset resistor in the indepen-
dent channels mode, only the two sense resistors are
used:
Selection of R
R
Selection of R
R
R
SENSE
SENSE
SENSE
SENSE
pull
up
1 = 72mV / I max ch 1
1 = 72mV / I max ch 2
being the value of the resistors connecting the
2 = 72mV / I max slave
1 = 72mV / I max master
K (
)
SENSE
SENSE
SENSE
SENSE
2
1 .
1 for the channel 1: (mohms)
2 for the channel 2: (mohms)
1 for the master channel: (in m ohm)
2 for the slave channel: (in m ohm)
X
5 .
5
V
V
OUT
V
SLAVE
OUT
1 .
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SC1175