sc1486aitstr Semtech Corporation, sc1486aitstr Datasheet - Page 9

• Manufacturer Part Number: sc1486aitstr
• Description: Complete Ddr1/2/3 Power Supply Controller
• Manufacturer: Semtech Corporation

+5V Bias Supplies
The SC1486A requires an external +5V bias supply in
addition to the battery. If stand-alone capability is
required, the +5V supply can be generated with an
external linear regulator. To minimize channel to channel
crosstalk, each controller has 4 supply pins, VDDP, PGND,
VCCA and VSSA.
To avoid interference between outputs, each controller
has its own ground reference, VSSA, which should be
tied by a single trace to PGND at the negative terminal of
that controller’s output capacitor (see Layout Guidelines).
All external components referenced to VSSA in the
schematic should be connected to the appropriate VSSA
trace. The supply decoupling capacitor for controller 1
should be tied between VCCA1 and VSSA1. Likewise, the
supply decoupling capacitor for controller 2 should be
tied between VCCA2 and VSSA2. A 10
be used to decouple each VCCA supply from the main
VDDP supplies. PGND can then be a separate plane which
is not used for routing traces. All PGND connections are
connected directly to the ground plane with special
attention given to avoiding indirect connections which
may create ground loops. As mentioned above, VSSA1
and VSSA2 must be connected to the PGND plane at
the negative terminal of their respective output
capacitors only. The VDDP1 and VDDP2 inputs provide
power to the upper and lower gate drivers. A decoupling
capacitor for each supply is required. No series resistor
between VDDP and 5V is required. See layout guidelines
for more details.
Pseudo-fixed Frequency Constant On-Time PWM
Controller
The PWM control architecture consists of a constant on-
time, pseudo fixed frequency PWM controller (see Figure
1, SC1486A Block Diagram). The output ripple voltage
developed across the output filter capacitor’s ESR
provides the PWM ramp signal eliminating the need for a
current sense resistor. The high-side switch on-time is
determined by a one-shot whose period is directly
proportional to output voltage and inversely proportional
to input voltage. A second one-shot sets the minimum
off-time which is typically 400ns.
On-Time One-Shot (t
The on-time one-shot comparator has two inputs. One
input looks at the output voltage, while the other input
samples the input voltage and converts it to a current.
POWER MANAGEMENT
Application Information
2006 Semtech Corp.
ON
)
resistor should
9
This input voltage-proportional current is used to charge
an internal on-time capacitor. The on-time is the time
required for the voltage on this capacitor to charge from
zero volts to VOUT, thereby making the on-time of the
high-side switch directly proportional to output voltage
and inversely proportional to input voltage. This
implementation results in a nearly constant switching
frequency without the need for a clock generator.
For VOUT < 3.3V:
For 3.3V
R
TON pin. Due to the high impedance of this resistor, the
TON pin should always be bypassed to VSSA using a 1nF
ceramic capacitor.
Enable & Psave
The EN/PSV pin enables the VDDQ (2.5V or 1.8V) supply.
REFIN and VDDP2 enable the VTT (1.25V or 0.9V)
supply. The VTT and VDDQ supplies may be enabled
independently, however it is usual to use a resistor
divider from VDDQ to generate REFIN, so if VDDQ is not
present, VTT will not be present.
When EN/PSV1 is tied to VCCA the VDDQ controller is
enabled and power save will also be enabled. When the
EN/PSV pin is tri-stated, an internal pull-up will activate
the VDDQ controller and power save will be disabled. If
PSAVE is enabled, the SC1486A PSAVE comparator will
look for the inductor current to cross zero on eight
consecutive switching cycles by comparing the phase
node (LX) to PGND. Once observed, the controller will
enter power save and turn off the low side MOSFET when
the current crosses zero. To improve light-load efficiency
and add hysteresis, the on-time is increased by 50% in
power save. The efficiency improvement at light-loads
more than offsets the disadvantage of slightly higher
output ripple. If the inductor current does not cross zero
on any switching cycle, the controller will immediately exit
power save. Since the controller counts zero crossings,
the converter can sink current as long as the current
does not cross zero on eight consecutive cycles. This
allows the output voltage to recover quickly in response
to negative load steps even when psave is enabled.
t
t
ON
ON
TON
is a resistor connected from the input supply to the
3
. 0
3 .
85
x
10
VOUT
3
3 .
12
x
10
R (
5V:
TON
12
R (
37
TON
x
10
37
3
)
x
10
V
V
3
OUT
SC1486A
IN
)
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V
V
OUT
50
IN
ns
50
ns