TPS54341_15 datasheet(23/48 Pages) TI

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RESR

COUT

gmps

Adc

COMP

gmea

350 mA/V

0.8 V

Power Stage

12 A/V

RESR

COUT

TPS54341

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SLVSC61 – NOVEMBER 2013

DETAILED DESCRIPTION (continued)

Figure 42. Small-Signal Model for Loop Response

Simple Small-Signal Model for Peak-Current-Mode Control

Figure 43 describes a simple small-signal model used to design the frequency compensation. The TPS54341

power stage is approximated by a voltage-controlled current source (duty-cycle modulator) supplying current to

the output capacitor and load resistor. The control to output transfer function is shown in Equation 14 and

consists of a DC gain, one dominant pole, and one equivalent-series-resistor (ESR) zero. The quotient of the

change in switch current and the change in COMP pin voltage (node c in Figure 42) is the power stage

transconductance, gmPS. The gmPS for the TPS54341 device is 16 A/V. The low-frequency gain of the power

stage is the product of the transconductance and the load resistance as shown in Equation 15.

As the load current increases and decreases, the low-frequency gain decreases and increases, respectively. This

variation with the load seems problematic at first glance, but fortunately the dominant pole moves with the load

current (see Equation 16). The combined effect is highlighted by the dashed line in the right half of Figure 43. As

the load current decreases, the gain increases and the pole frequency lowers, keeping the 0-dB crossover

frequency the same with varying load conditions. The type of output capacitor chosen determines whether the

ESR zero has a profound effect on the frequency compensation design. Using high-ESR aluminum-electrolytic

capacitors can reduce the number frequency compensation components required to stabilize the overall loop

because the phase margin increases by the ESR zero of the output capacitor (see FUNCTIONAL BLOCK

DIAGRAM).

Figure 43. Simple Small-Signal Model and Frequency Response for Peak-Current-Mode Control

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TPS54341 Datasheet(PDF) 23 Page - Texas Instruments