LT7184SRVPBF datasheet(36/52 Pages) AD

36 / 52 page

Data Sheet

LT7184S

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Rev. 0

36 of 52

ILOADNEGMAX − ΔIL/2 ≥ ILIM _NEG(MAX)

(5)

For example, in the current limit range 2 with a negative current limit typical of -7.5A (max -6.5A) and ripple current

of 2A, the most negative load that the LT7184S supports is -5.5A. A larger negative current load than that will risk an

output overvoltage. When operating with a negative load, the current gets sunk at the output voltage, VOUT, and is

pushed out of the PVIN input pin. The supply connected to PVIN must be able to absorb this excess current, or an

overvoltage at PVIN will occur.

An inductor must be chosen with a saturation current (typically labeled ISAT) higher than the maximum peak current

when operating in the current limit.

IL (PEAK MAX) = ILIM _POS + ΔIL

(6)

To avoid overheating and poor efficiency, an inductor must be chosen with an RMS current rating that is greater than

the maximum expected output load of the application. Preferably, the inductor RMS rating supports the average

inductor current in the current limit.

IL (AVG MAX) = ILIM (POS) +

ΔIL

(7)

If the inductor current rating does not support IL(AVGMAX) described in Equation 7, either select a suitable current limit

or set the VOUT_UV_FAULT_RESPONSE command to disable the channel if the output voltage falls below the

VOUT_UV_FAULT_LIMIT. The default setting of VOUT_UV_FAULT_RESPONSE is to continue operation at the valley

current limit while the VOUT_UV fault condition is present. For the highest efficiency, the inductor series resistance

(DCR or ESR) should be minimized, and its core material should be intended for high-frequency applications.

Input and Output Capacitors

The Low ESR ceramic capacitors should be used at both the output and input supplies of the switching regulators.

X5R or X7R ceramic capacitors are recommended for best performance overtemperature and applied voltage. Ensure

that the selected capacitor’s voltage ratings exceed the maximum application voltages.

Decouple the PVIN pins with low ESL and ESR ceramic capacitors as close as possible to the two PVIN pins with

returns to the appropriate ground return pins, as shown in Table 7 and bulk ceramic capacitors to support the input

ripple current.

See the Typical Applications section in this data sheet for suggested output capacitor values. The output capacitor

values must be selected to maintain stability over selected operating conditions including operating frequency,

compensation (gMEA, and compensation network, RITH, and CITH), as well as the programmed current limit, which

selects the modulator transconductance.

Programmable PWM Control Loop Compensation

The LT7184S supports internal or external PWM control loop compensation, as shown in Figure 37 and Figure 38. For

single-phase applications, internal compensation is selected by tying the channel's ITH pin to INTVCC. The PolyPhase

operation requires an external compensation, and the ITH pins of all the PolyPhase channels must be connected to

an external compensation network.

Control loop compensation can be programmed using the MFR_PWM_MODE_LT7184S command. In either internal

or external compensation, the transconductance of the LT7184S PWM error amplifier can be adjusted using

MFR_PWM_MODE_LT7184S buts [13:11], as shown in Table 13. When internal compensation is selected, the internal

PWM loop compensation resistor, RITH, of the LT7184S can be adjusted in non-linear increments from 5kΩ to 60kΩ

(typical) using the MFR_PWM_MODE_LT7184S buts [5:3], as shown in Table 14. The internal compensation capacitor,

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