LTC3445 datasheet(13/24 Pages) LINER

13 / 24 page

LTC3445

3445fa

micro-steps. The table below shows the register settings

and corresponding slew rates.

REG1 [1:0]

SLEW RATE (mV/

µs)

11.3

7.5

3.8

0.9

It should be noted that during DAC transistions, PGOOD

fault reporting is disabled.

LDO OPERATION

Adjustable Operation

The LTC3445 contains two 50mA LDOs with an output

voltage range of 0.6V to (VCC2 – 0.3V). The output voltage

is set by the ratio of two external resistors as shown in

Figure 1. Each LDO servos the output voltage (Pin LDOx)

in order to maintain a feedback voltage (Pin LDOxFB) of

0.6V. The current in R1 and R2 is then equal to 0.6V/R2.

The regulated voltage is equal to:

VOUT = (0.6V/R2) • (R1+R2)

Frequency Compensation

The LT3445 is frequency compensated by an internal

dominant pole. An output capacitor of 2

µF to 10µF is

usually large enough to provide good stability. In order to

insure stability, a feedforward capacitor may be needed

between the output pin and the feedback pin. This cancels

the pole formed by the stray capacitance in large value

feedback resistors. Also, a feedback capacitor minimizes

noise pickup and improves ripple rejection.

PowerPath OPERATION

The output of the PowerPath (VCC BATT) is controlled by

a combination of three inputs: main battery (VCC1), VTRACK,

and VBACKUP.

OPERATIO (refer to Figure 1)

SPR = 00 (Spread Spectrum OFF)

NOISE

10dBm/DIV

SPR = 01

NOISE

10dBm/DIV

START FREQ: 100kHz, RBW: 10kHz, STOP FREQ: 30MHz

SPR = 10

NOISE

10dBm/DIV

SPR = 11

NOISE

10dBm/DIV

START FREQ: 100kHz, RBW: 10kHz, STOP FREQ: 30MHz

Figure 4. LTC3445 Output Noise Spectrum

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