LM2745 datasheet(21/37 Pages) TI

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100

10k

100k

FREQUENCY (Hz)

-150

-120

-90

-60

-30

100

10k

100k

FREQUENCY (Hz)

-60

-44

-28

-12

VIN x RO

GPS =

VRAMP

sCORC + 1

+ bs + c

fESR =

2SCOESR

= 20.3 kHz

= 4.5 kHz

RO + RL

fDP =

LCO(RO + ESR)

VIN

ADC =

VRAMP

3.3

1.0

= 10.4 dB

LM2745, LM2748

www.ti.com

SNOSAL2E – APRIL 2005 – REVISED APRIL 2013

The power stage modulator provides a DC gain ADC that is equal to the input voltage divided by the peak-to-peak

value of the PWM ramp. This ramp is 1.0Vpk-pk for the LM2745/8. The inductor and output capacitor create a

double pole at frequency fDP, and the capacitor ESR and capacitance create a single zero at frequency fESR. For

this example, with VIN = 3.3V, these quantities are:

In the equation for fDP, the variable RL is the power stage resistance, and represents the inductor DCR plus the

on resistance of the top power MOSFET. RO is the output voltage divided by output current. The power stage

transfer function GPS is given by the following equation, and Figure 30 shows Bode plots of the phase and gain in

this example.

a = LCO(RO + RC)

b = L + CO(RORL + RORC + RCRL)

c = RO + RL

Figure 30. Power Stage Gain and Phase

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