SC470 datasheet(14/31 Pages) SEMTECH

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SC470

POWER MANAGEMENT

Application Information (Cont.)

This calculation assumes the absolute worst-case

condition of a full-load to no-load step transient occurring

when the inductor current is at its highest. The

capacitance required for smaller transient steps may be

calculated by substituting the desired current for the I

OUT

term.

For our example:

OUT(MIN) = 595µF.

We will select 440µF, using two 220µF, 25m

Ω

capacitors in parallel. For smaller load release overshoot,

660µF may be used.

Next we calculate the RMS input ripple current, which is

largest at the minimum battery voltage:

()

RMS

MIN

BAT

OUT

)

MIN

(

BAT

OUT

)

RMS

(

•

−

•

For our example:

IN(RMS) = 2.14ARMS

Input capacitors should be selected with sufficient ripple

current rating for this RMS current, for example a 10µF,

1210 size, 25V ceramic capacitor can handle a little more

than 2A

RMS (Refer to manufacturer’s data sheets).

Finally, we calculate the current limit resistor value. As

described in the current limit section, the current limit

looks at the “valley current”, which is the average output

current minus half the ripple current. We use the

maximum room temperature specification for MOSFET

DS(ON) at VGS = 4.5V for purposes of this calculation:

)

MIN

(

VBAT

RIPPLE

OUT

VALLEY

−

The ripple at low battery voltage is used because we want

to make sure that current limit does not occur under

normal operating conditions.

()

Ohms

)

(

VALLEY

ILIM

−

•

For our example:

VALLEY = 5.13A, RDS(ON) = 9mΩ and RILIM = 7.76kΩ

We select the next lowest 1% resistor value: 7.68k

Ω

Adding an Additional Output Voltage For Dynamic

Voltage Switching

If we design this output to be capable of dynamically

switching between 1.2V and 1.0V, then we would repeat

these calculations to determine if any components need

changing. The 1.0V output suggests a value for C

TOP of

82pF, but the value of 56pF required by the 1.2V design

should work fine, and can always be increased if neces-

sary. Also, the current limit resistor required is slightly

higher: R

ILIM = 7.87kΩ. The higher value should be used.

Lastly, the bottom feedback resistor, R

BOT will need to

change to 20.0k

Ω. The schematic in Figure 8 on Page

17 shows the complete design.

Dynamically Switching Output Voltages

It is important to note that in order for dynamic output

voltage switching to work, the SC470 must be in

Continuous Conduction Mode (EN/PSV = floating) when

transitioning from V

OUT(HIGH) to VOUT(LOW). Otherwise the

SC470 has no means to discharge the output voltage

and may OVP and latch off when this transition is initiated

(depending upon the difference between the two

voltages). If CCM is on, the SC470 will actively discharge

the output down to the correct voltage.

Dynamically switching output voltages is very easy,

requiring one switch to add or remove an additional

resistor in parallel to the bottom feedback resistor. Ideally,

the resistor will be switched using an open drain output

from another IC, as shown in Figure 4.

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