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型號(hào)：	RC5501
文件頁(yè)數(shù)：	770/1328頁(yè)
文件大?。?/td>	9519K
代理商：	RC5501

It is important to note that the softstart pin is not an enable;

pulling it low will not necessarily turn off all outputs.

In main power operation, the RC5060 is run from the +12V

main supply. This supply also provides voltage to the various

MOSFET gates. However, during standby, this supply is off.

To provide power to the chip and the appropriate gates, the

RC5060 incorporates a free-running charge pump. As shown

in Figures 3 and 4, and in the block diagram on the front

page, a capacitor attached between pins 1 and 2 of the RC5060

acts as a charge pump with internal diodes. The charge pump

output is internally diode or’red with the 12V input. The 12V

input must have a series diode to prevent back-feeding the

charge pump to the + 12V main when in standby. The 12V

input line needs a bypass capacitor for high-frequency noise

The RC5060 does not directly detect current through the

eight devices that power its outputs. Instead, it monitors the

four output voltages. In the event of a hard short, the voltage

drops below 80% of nominal, and all outputs are latched off,

and remain off until 5V standby power is recycled. The over-

current latch off is delayed by 30μsec to prevent nuisance trips.

During softstart, the overcurrent voltage monitors are kept

proportional to the reference, to avoid tripping overcurrent

during startup. The monitors are kept active during softstart,

to avoid turning on into a short.

In the S5 state, when the memory outputs are off, the voltage

monitors on the memory lines are disabled, to prevent trip-

ping the overcurrent. When turning these lines back on from

the S5 state, the delay prevents overcurrent from tripping.

If the 2.5V dual is not used, its feedback line, pin 15, must be

connected to 5V dual as shown in Figure 4, to prevent an

overcurrent trip.

If the +5V standby is below approximately 4.5V, the RC5060

will leave off or turn off all outputs. Similar comments apply

to the +12V main at 7.5V. The +5V standby UVLO has

approximately 0.5V hysteresis, the +12V main UVLO 1V.

The Power Good is an open collector that pulls low if any of

the outputs are less than 80% of nominal.

Over Temperature

The RC5060 is capable of sourcing substantial current, 200mA

minimum to the RAMBUS transistor’s base during S0, 144mA

to the RAMBUS line during S3, and 100mA to SDRAM

during S3. As a result, there can be heavy power dissipation

in the IC. While the RC5060 is designed to accept this power

dissipation, any overloading of outputs can cause excessive

heating. If the RC5060 die temperature exceeds about 150°,

all outputs are shut off. Outputs remain off until the die tem-

perature returns to its safe area.

Transistor Selection

External transistor selection depends on usage, differing for

the linear regulators and the switches.

The MOSFET switches, Ql, Q3, Q5 and Q6 should be sized

based on regulation requirements and power dissipation.

Since the ATX outputs are ±5%, the outputs driven from

them must be wider. As an example, if we want to hold 3.3V

SDRAM to -10%, we can drop only 5% = 165mV across Q1.

At 4.8A, this means Ql must have a maximum R

165mV/4.8A = 34m

, including tolerance and self-heating

effects. We thus choose a Fairchild FDS4410Y, which has

at 25°C. We can esti-

mate power dissipation as (4.8A)

= 460mW, which

should be acceptable for this package. Similar calculations

apply to the other MOSFET switches.

Q4 is a MOSFET functioning as a linear regulator. Since it

delivers only 500mA, it is easy to select a MOSFET, it need

less than (5V–5%–3.3V)/500mA = 2.9

We select the Fairchild NDS9956A which has maximum

at 25°C. Power dissipation

will be a maximum of (0.5A)

Q2 is an NPN bipolar functioning as a linear regulator. As

already discussed, it must have a V

lower than 635mV

= 200mA. Its power dissipation can be as

high as (3.3V + 5%–2.5V) * 2A = l.9W.

Output Capacitor Selection

Output capacitor selection depends on whether the line has

overlap time or not.

For both the 5V dual and the 2.5V dual, there is guaranteed

overlap time between when one source is turned on and the

other source turned off. For these outputs, the output capaci-

tor is not needed to hold up the supply, but only for noise fil-

tering and to respond to transient loading.

The 3.3V dual and 3.3V SDRAM outputs have deadtime

between when one source is turned off and the other source

turned on. During the time when both are off, the output cur-

rent must be supplied by the output capacitor. Mitigating

this, it must be realized that the system will be designed in

such a way that the current has gone to its sleep value before

the transition occurs. For example, the 3.3V dual has a sleep

current of 500mA maximum. Maximum deadtime is 6μsec,

and so charge depletion is 500mA * 6μsec = 3μC. Suppose

that we have a total of 8% drop due to the source tolerance

and the MOSFET drop, and we are trying to hold 10% regu-

lation. The remaining 2% = 66mV implies a minimum

capacitance of 3μC/66mV = 45μF.

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RC5503	制造商:未知廠家制造商全稱:未知廠家功能描述:UHF Controller

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