TPS2399DMT7G datasheet(13/15 Pages) ONSEMI

13 / 15 page

TPS2398, TPS2399

www.onsemi.com

For the TPS2398 and TPS2399, the typical duration of the

soft−start ramp period, tSS, is given by Equation 3.

SS + 1183 @ CIRAMP

(eq. 3)

Where:

• tSS is the soft−start period in ms, and

• CIRAMP is given in mF

During this current ramp period, the load voltage magnitude

which is attained is estimated by Equation 4.

LSS +

AVG

@ C

LOAD @ CRAMP @ 100 @ RSNS

@ t

(eq. 4)

Where:

• VLSS is the load voltage reached during soft−start,

• iAVG is 3.38 mA for the TPS2398 and TPS2399,

• CLOAD is the amount of the load capacitance, and

• tSS is the soft−start period, in seconds

The quantity iAVG in Equation 4 is a weighted average of the

two charge currents applied to CIRAMP during turn−on,

considering the typical output values.

If the result of Equation 4 is larger than the maximum

input supply value, then the load can be expected to charge

completely during the inrush slewing portion of the insertion

event. However, if this voltage is less than the maximum

supply input, VIN(MAX), the HSPM transitions to the

constant−current charging of the load. The remaining

amount of time required at IMAX is determined from

Equation 5.

CC +

LOAD @ VIN(MAX) * VLSS

REF_K(MIN)

SNS

(eq. 5)

Where:

• tCC is the constant−current voltage ramp time, in

seconds, and

• VREF_K(MIN) is the minimum clamp voltage, 33 mV.

With this information, the minimum recommended value

timing capacitor CFLTTM can be determined. The delay time

needed will be either a time tSS2 or the sum of tSS2 and tCC,

according to the estimated time to charge the load. The

quantity tSS2 is the duration of the normal rate current ramp

period, and is given by Equation 6.

SS2 + 0.35 @ CRAMP

(eq. 6)

Where:

• CRAMP is given in microfarads

Since fault timing is generated by the constant−current

charging of CFLTTM, the capacitor value is determined from

either Equation 7 or 8, as appropriate.

FLTTM(MIN) +

@ t

SS2

3.75

(eq. 7)

(eq. 8)

FLTTM(MIN) +

@ t

SS2 ) tCC

3.75

Where:

• CFLTTM(MIN) is the recommended capacitor value, in

microfarads,

• tSS2 is the result of Equation 6, in seconds, and

• tCC is the result of Equation 5, in seconds.

Continuing the typical application example, using a 100

input capacitor (CLOAD), Equations 3 and 4 estimate the

load voltage ramping to approximately −46 V during the

soft−start period. If the module should operate down to

−72 V input supply, approximately another 1.58 ms of

constant−current charging may be required. Therefore,

Equations 6 and 8 are used to determine CFLTTM(MIN), and

the result of 43 nF suggests the 47 nF standard value.

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