
RC5060
10
P
If the SLP_S5# line is asserted, the 3.3V SDRAM output is
on. In this condition, if either the
SLP_S3#
or the PWROK
line, or both, are de-asserted, the linear regulator is on and
the MOSFET is off. Only in the case if both the
SLP_S3#
and the PWROK lines are asserted, the MOSFET is on and
the linear regulator is off.
In a typical system, it is anticipated that standby current will
be about 100mA maximum. Full power current will be as
high as 4.8A maximum, so that Ql must have a low R
DS,on
in order to prevent excessive voltage drop across it.
2.5V Dual Output
The 2.5V dual output is intended to provide power to RAM-
BUS memory. Only high-end systems will use this power.
Those systems using RAMBUS may also use the SDRAM
power, possibly piped to the same slots, to ensure backward
compatibility or even mixed operation of SDRAM with
RAMBUS.
2.5V dual is generated by one external NPN bipolar acting as
a linear regulator from +3.3V main, and one linear regulator
internal to the RC5060 from +5V standby, as shown in Fig-
ure 3, and in the block diagram on the front page. When
main power is present, the NPN Q2 linear regulates, and
when main power is absent, the internal linear regulator is
on. Q2 cannot be substituted with a MOSFET. If used in one
direction, the MOSFET’s body diode would permit back-
feed; if used in the other direction, it would short-circuit the
linear regulator action.
2.5V dual output is controlled in the same way and by the
same lines as the 3.3V SDRAM output. In a typical system,
it is anticipated that standby current will be a maximum of
144mA, and full-power current may be as high as 2A. This
places some significant constraints on the selection of Q2.
Since its input may be as low as (3.3V - 5%) = 3.135V, there
is only 3.135V -2.5V = 635mV of V
CE
headroom for its
operation as a linear regulator. For this reason the RC5060
can provide up to 200mA of steady-state base current. The
TIP41 device shown has a sufficiently low V
CE, sat
to guaran-
tee worst-case regulation even at 2A I
E
with this base cur-
rent.
RC5060 ACPI Control Lines
As already discussed, the RC5060 outputs are controlled by
the three ACPI control lines,
SLP_S3#
, SLP_S5# and
PWROK, as summarized in Table 1. System designers must
in particular be careful to ensure that their system is designed
with SLP_S5#, not SLP_S5#; if SLP_S5 is used, it must be
inverted before being used with the RC5060.
The control lines have internal pull-ups of approximately
10μA, and so can be controlled by open collector drivers if
desired. In a noisy system, it may be desirable to filter these
lines, which can be done with a 1K
resistor and a small
capacitor.
RC5060 Dynamic Operation
The RC5060 is designed to minimize the output capacitance
required to hold up the various output lines during transitions
between different states. Thus in particular, the 5V dual and
2.5V dual outputs have guaranteed minimum overlap times,
the time (as shown in Figure 2) during a state transition dur-
ing which both main and standby are connected to the out-
put. This overlap time guarantees that a power source is
always connected to the output, so that there will be no dip in
the output voltage during state transitions. There is also a
maximum overlap time, to ensure that the standby power
doesn’t have to source main power very long, thus minimiz-
ing thermal stress on the standby device.
The 3.3V dual and 3.3V SDRAM are different than the other
outputs, because they are powered by both a linear regulator
and a switch. If the linear regulator were to turn on while the
switch is on (or vice versa) the linear regulator would supply
power to the main line through the switch. For this reason,
the linear regulator must be off before the switch is on, and
vice versa. Thus, these two outputs have guaranteed minimum
deadtime when both linear regulator and switch are off. Dur-
ing this time, the output capacitors must hold up the load,
and so there is also a specified maximum deadtime, allowing
a maximum necessary capacitance to be selected, see below.
Stability
As with all linear regulators, the RC5060’s linear regulators
require a minimum load. With the exception of the 3.3V dual
output, however, all of these minimum loads are internal to
the RC5060. The 3.3V dual output requires a minimum load
of 50mA; if a situation may occur in which the load is less than
50mA, additional steps may be necessary to ensure stability.
Furthermore, depending on location, it may be necessary to
bypass the drain (or collector) of the linear regulator with a
low ESR capacitor for stability. As a rule of thumb, if the
pass element is more than 1” from its power source, it should
have a bypass.
Softstart
Pin 13 of the RC5060 functions as a softstart. When power is
first applied to the chip, a constant current is applied from
the pin into an external capacitor, linearly ramping up the
voltage. This ramp in turn controls the internal reference of
the RC5060. providing a softstart for the linear regulators.
The actual state of the RC5060 on power up will be deter-
mined by the state of its control lines.
The switches in the system must be either on or off, and so
softstart has no effect on their characteristics: if the appropri-
ate control signals are asserted, they will turn on at once.
The softstart is effective only during power on. During a
transition between states, such as from S5
→
S0, the linear
regulators are not softstarted.