SC453TSTRT datasheet(16/26 Pages) SEMTECH

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POWER MANAGEMENT

SC453

The second condition occurs later after the initial tran-

sient. Due to the fast response of the hysteretic convert-

ers, less then < 100ns, the capacitor voltage does not

droop very far before the inductor current starts ramping

up. Once the inductor current starts to rise, the I•ESR

comp onent of the capacitor voltage start to move upward.

At the same time, the capacitive discharging causes the

capacitor voltage to move downward. The lowest output

voltage is reached when the inductor current reaches the

load current (note the shaded area on the graph). If this

voltage is higher than the lower limit (VPOS_TRANS), then

the transient response passes.

Since the highest output voltage has the most severe re-

quirements, any other modes are satisfied by a design op-

timized for the highest output voltage.

For the second condition, the inductor value is deter-

mined, which is a function of the highest desired switching

frequency. The maximum frequency occurs at the highest

input voltage. As a reasonable compromise between ef-

ficiency and component size, a maximum switching fre-

quency of 350kHz is desired.

The design is based on the following three factors: 1) mini-

mum inductance requirement; 2) low DCR; 3) height and

package size consideration. The design choice is made

based upon the requirement of the converter design, in-

cluding minimum inductance, minimum saturation cur-

rent, efficiency, foot area, maximum allowable height, and

device availability. In the current demo board design,

This value of inductance is required up to maximum load.

Inductors with a swinging choke characteristic, where the

zero current value of inductance is much less than the

full load current inductance can be used, as long as the

above restriction is met. Then, the worst-case (low input

voltage) response time (the time for the current to reach

the new transient value) is:

Add 100ns for the typical propagation delay from a change

at the output to the MOSFET switch turning on in reaction.

Since the shaded area is, the total charge taken out of the

capacitor is

∆Q = C / dV = (dI/dt)/2. This is calculated

using the following equation.

CMINP

IMAX_FL ILKGMAX

−

() dT 1 10 7

−

⋅

sec

⋅

(

)

⋅

VPOS_TRANS

CMINP

4.278

−

This condition applies only to the positive transient.

LMIN

dMIN

VINMAX VMAX_NL

−

() ESR

MAX

RCS

()

⋅

FS VRIPPLE

⋅

ESR MAX RCS

ESR MAX

⎛

⎜

⎝

⎞

⎠

⋅

LMIN

5.422

−

dMIN

VMAX_NL

VINMAX

Applications Information (Cont.)

ESR MAX

VPOS_TRANS

IMAX_FL ILKGMAX

−

(

)

0.60

μH

⋅

350 K Hz

⋅

L1 IMAX_FL ILKGMAX

−

()

⋅

VINMIN

VMAX_NL

−

1.326

−

ESR MAX 3.333

−

×Ω

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