LT7184S datasheet(35/52 Pages) AD

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Data Sheet

LT7184S

analog.com

Rev. 0

35 of 52

As shown in Figure 36, when negative valley current limit occurs (sinking output current due to the output being

pulled up externally), the inductor valley current is ILIM-NEG, the average output current is ILIM-NEG + ΔIL/2, and the peak

inductor current is ILIM-NEG + ΔIL.

Figure 36.ILIM(NEG)

The LT7184S offers four settings for the valley's current limits. The valley current limit selection is controlled by

MFR_PWM_MODE_LT7184S bits [10:9], as shown in Table 12. The factory default current limit setting is 9A (typical)

positive valley current limit and -7.5A (typical) negative valley current limit. Note that the modulator's current sense

gain (dIOUT/dVITH) is also changed as the current limit selection is changed, which must be considered when optimizing

control loop compensation. The valley Current Limits are affected by VOUT/L (V/µH) and operating frequency. See the

Typical Performance Characteristics for more information.

Table 12. Valley Current Limit Selection: MFR_PWM_MODE_LT7184S bits [10:9]

VALUE

POSITIVE VALLEY

CURRENT LIMIT:

ILIM-POS (TYP)

NEGATIVE VALLEY

CURRENT LIMIT:

ILIM-NEG (TYP)

CURRENT SENSE

TRANSCONDUCTANCE:

gM(MOD) = dIOUT/dVITH (TYP)

-9A

20.0 A/V

2 (default)

-7.5A

18.33 (A/V)

-6A

14.44 (A/V)

-3.5A

8.33 (A/V)

Inductor Selection

For a given application’s input voltage and output voltage, the inductor value and operating frequency determine

the ripple current, as defined in Equation 3.

ΔIL =

VOUT

fSW×L

(1 −

VOUT

PVIN

)

(3)

Lower ripple current reduces losses in the inductor and ESR losses in the output capacitors and reduces output

voltage ripple. The highest efficiency operation is obtained at low frequency with a small ripple current. However,

achieving this requires a large inductor.

A reasonable starting point is to choose a ripple current of about 50% of IOUT(MAX), up to a maximum of about 4A. To

guarantee that the ripple current does not exceed a specified maximum, the inductance should be chosen according

to the following:

L =

VOUT

fSW×ΔIL MAX

(1 −

VOUT

PVIN

)

(4).

The inductor ripple current (ΔIL),and maximum negative load current (ILOADNEGMAX) must be chosen such that the most

negative inductor valley current limit is above the maximum (least negative) valley current limit indicated in the

Electrical Characteristics (See Table 3), or an output overvoltage will occur.

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