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LTC3554

3554f

Holding ON low through the one second power-down

period will not cause a power-up event at end of the one

second period. ON must be brought high following the

power-down event and then go low again for 400ms to

establish a valid power-up event, as shown in Figure 12.

The regulators in Figure 13 are slewing up with nominal

output capacitors and no-load. Adding a load or increas-

ing output capacitance on any of the outputs will reduce

the slew rate and lengthen the time it takes the regulator

to get into regulation.

OPERATION

BAT

VBUS

ON (PB)

PBSTAT

BUCK1

BUCK2

PGOOD

PWR_ON1

PWR_ON2

STATE

PON

PUP1

PDN1

3554 TD06

400ms

50ms

VOUT2

0.5V/DIV

VOUT1

1V/DIV

3554 F13

100μs/DIV

Figure 13. Power-Up Sequencing

Figure 12. Hard Reset Via Holding ON Low for 5s

LAYOUT AND THERMAL CONSIDERATIONS

Printed Circuit Board Power Dissipation

In order to be able to deliver maximum charge current

under all conditions, it is critical that the Exposed Pad

on the backside of the LTC3554 package is soldered

to a ground plane on the board. Correctly soldered to a

2500mm2 ground plane on a double-sided 1oz copper

board, the LTC3554 has a thermal resistance (θJA) of

approximately 70°C/W. Failure to make good thermal

contact between the Exposed Pad on the backside of the

package and an adequately sized ground plane will result

in thermal resistances far greater than 70°C/W.

The conditions that cause the LTC3554 to reduce charge

current due to the thermal protection feedback can be

approximated by considering the power dissipated in the

part. For high charge currents the LTC3554 power dis-

sipation is approximately:

PD = (VBUS–BAT) • IBAT + PD(REGS)

where PD is the total power dissipated, VBUS is the supply

voltage, BAT is the battery voltage, and IBAT is the battery

charge current. PD(REGS) is the sum of power dissipated

on chip by the step-down switching regulators.

Power-Up Sequencing

Figure 13 shows the actual power-up sequencing of the

LTC3554. Buck1 and buck2 are both initially disabled (0V).

Once the pushbutton has been applied (ON low) for 400ms

buck1 is enabled. Buck1 slews up and enters regulation.

The actual slew rate is controlled by the soft start func-

tion of buck1 in conjunction with output capacitance and

load (see the Step-Down Switching Regulator Operation

section for more information). When buck1 is within about

8% of ﬁnal regulation, buck2 is enabled and slews up

into regulation. 230ms after buck2 is within 8% of ﬁnal

regulation, the PGOOD output will go high impedance.

LTC3554EPD Datasheet(PDF) 31 Page - Linear Technology

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