LTC3445_15 Datasheet, PDF (13/24 Pages) Linear Technology – I2C Controllable Buck Regulator with Two LDOs

U
OPERATIO (refer to Figure 1)
SPR = 00 (Spread Spectrum OFF)
LTC3445
SPR = 10
NOISE
10dBm/DIV
NOISE
10dBm/DIV
START FREQ: 100kHz, RBW: 10kHz, STOP FREQ: 30MHz
SPR = 01
START FREQ: 100kHz, RBW: 10kHz, STOP FREQ: 30MHz
SPR = 11
NOISE
10dBm/DIV
NOISE
10dBm/DIV
START FREQ: 100kHz, RBW: 10kHz, STOP FREQ: 30MHz
START FREQ: 100kHz, RBW: 10kHz, STOP FREQ: 30MHz
Figure 4. LTC3445 Output Noise Spectrum
micro-steps. The table below shows the register settings
and corresponding slew rates.
REG1 [1:0]
00
01
10
11
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 (V_CC2 – 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:
V_OUT = (0.6V/R2) • (R1+R2)
Frequency Compensation
The LT3445 is frequency compensated by an internal
dominant pole. An output capacitor of 2µF 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 (V_CC BATT) is controlled by
a combination of three inputs: main battery (V_CC1), V_TRACK,
and V_BACKUP.
3445fa
13