SC453TSTRT datasheet(15/23 Pages) SEMTECH

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POWER MANAGEMENT

SC453

In the second condition, because the hysteretic converter

responds in < 100ns, the capacitor does not droop very

far before the inductor current starts ramping up. (This is

not true of control schemes where time constants in the

error ampliﬁer cause delays.) Once the inductor current

starts to rise, the increasing ∆V of the capacitor is offset

by reduced ∆V from the ESR, so ∆V is constant. If the ∆V

due to the charge taken from the capacitor before the in-

ductor current reaches the load current (note the shaded

area on the graph) is less than 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, we need to know the inductor

value, which is a function of the highest desired switching

frequency. The maximum frequency occurs at the high-

est input voltage. As a reasonable compromise between

efﬁciency and component size, a maximum switching fre-

quency of 350kHz is desired.

Based on the following four factors: 1) minimum induc-

tance requirement; 2) device availability at the time we

designed the evaluation board; 3) low DCR ; 4) height and

package size consideration. Keep in mind, the choice you

make should be based upon the requirement of the con-

verter design, including minimum inductance, minimum

saturation current, efﬁciency, foot area, maximum allow-

able height, and of course, device availability. In the cur-

rent 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 propagation delay from a change at

the output to the MOSFET switch turning on in reaction.

Since the shaded area is triangular, the total charge taken

out of the capacitor = (dI / dt) / 2. Q = C / dV = (dI / dt) /

2, therefore;

CMINP

IMAX_FL ILKGMAX

−

() dT 1 10 7

−

⋅

sec

⋅

(

)

⋅

VPOS_TRANS

CMINP 4.278 10

−

This condition applies only to the positive transient.

LMIN dMIN

VINMAX VMAX_NL

−

() ESRMAX RCS

()

⋅

FS VRIPPLE

⋅

ESRMAX RCS

ESRMAX

⎛

⎜

⎝

⎞

⎠

⋅

LMIN 5.422 10

−

dMIN

VMAX_NL

VINMAX

Applications Information (Cont.)

ESRMAX

VPOS_TRANS

IMAX_FL ILKGMAX

−

(

)

ESRMAX 3.333 10

−

×Ω

L1 0.60 μH

⋅

FS 350K Hz

⋅

L1 IMAX_FL ILKGMAX

−

()

⋅

VINMIN VMAX_NL

−

dT 1.326

−

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