SC488 datasheet(14/24 Pages) SEMTECH

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POWER MANAGEMENT

SC488

For stability, place a 10Ω/1μF series combination from REF

to VSSA. If REF load capacitance exceeds 1μF, place at

least 10Ω’s in series with the load capacitance to prevent

instability. It is possible to use only one 10Ω resistor, by

connecting the load capacitors in parallel with the 1μF,

and connecting the load REF to the capacitor side of the

10Ω resistor. (See the Typical Application Circuit on Page

1.) Note that this resistor creates an error term when REF

has a DC load. In most applications this is not a concern

since the DC load on REF is negligible.

Design Procedure

Prior to designing a switching output and making com-

ponent selections, it is necessary to determine the input

voltage range and output voltage speciﬁcations. To dem-

onstrate the procedure, the output for the schematic in

Figure 7 on page 19 will be designed.

The maximum input voltage (V

BAT(MAX)) is determined by the

highest AC adaptor voltage. The minimum input voltage

BAT(MIN)) is determined by the lowest battery voltage af-

ter accounting for voltage drops due to connectors, fuses

and battery selector switches. For the purposes of this

design example we will use a VBAT range of 8V to 20V to

design VDDQ.

Four parameters are needed for the design:

Nominal output voltage, V

OUT.

We will use 1.8V with

internal feedback resistors (FB pin tied to VCCA).

Static (or DC) tolerance, TOL

ST (we will use +/-2%).

Transient tolerance, TOL

TR and size of transient (we

will use +/-8% for a 10A to 5A load release for this

demonstration).

Maximum output current, I

OUT (we will design for 10A).

Switching frequency determines the trade-off between

size and efﬁciency. Increased frequency increases the

switching losses in the MOSFETs, and losses are a func-

tion of VBAT2. Knowing the maximum input voltage and

budget for MOSFET switches usually dictates where the

design ends up. The default R

tON values of 1MΩ and

715kΩ are suggested only as a starting point.

The ﬁrst thing to do is to calculate the on-time, t

ON, at

BAT(MIN) and VBAT(MAX), since this depends only upon VBAT, VOUT

and Rt

ON.

)

MIN

(

BAT

OUT

tON

3.3

ON_VBAT(MI

and,

)

MAX

(

BAT

OUT

tON

3.3

ON_VBAT(MA

From these values of t

ON we can calculate the nominal

switching frequency as follows:

ON_VBAT(MI

BAT(MIN)

OUT

(MIN)

SW_VBAT

and,

ON_VBAT(MA

BAT(MAX)

OUT

(MAX)

SW_VBAT

ON is generated by a one-shot comparator that samples

BAT via RtON, converting this to a current. This current is

used to charge an internal 3.3pF capacitor to V

OUT. The

equations above reﬂect this along with any internal com-

ponents or delays that inﬂuence t

ON. For our example we

select R

tON = 1MΩ:

ON_VBAT(MIN) = 820ns and, tON_VBAT(MAX) = 358ns

SW_VBAT(MIN) = 274kHz and fSW_VBAT(MAX) = 251kHz

Now that we know t

ON we can calculate suitable values for

the inductor. To do this we select an acceptable inductor

ripple current. The calculations below assume 50% of I

OUT

which will give us a starting place.

OUT

0.5

(MIN)

ON_VBAT

OUT

BAT(MIN)

(MIN)

VBAT

and,

OUT

0.5

ON_VBAT(MA

OUT

BAT(MAX)

(MAX)

VBAT

For our example,

VBAT(MIN) = 1.02μH and LVBAT(MAX) = 1.30μH,

Application Information (Cont.)

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