LTC3554 LINER [Linear Technology], LTC3554 Datasheet - Page 31

• Manufacturer Part Number: LTC3554
• Description: Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators
• Manufacturer: LINER [Linear Technology]

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.
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 fi nal regulation, buck2 is enabled and slews up
into regulation. 230ms after buck2 is within 8% of fi nal
regulation, the PGOOD output will go high impedance.
OPERATION
PWR_ON1
PWR_ON2
STATE
ON (PB)
PBSTAT
PGOOD
BUCK1
BUCK2
Figure 12. Hard Reset Via Holding ON Low for 5s
V
BAT
BUS
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
PON
50ms
5s
PDN1
1s
HR
400ms
PUP1
3554 TD06
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.
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
2500mm
board, the LTC3554 has a thermal resistance (θ
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:
where P
voltage, BAT is the battery voltage, and I
charge current. P
on chip by the step-down switching regulators.
P
D
= (V
0.5V/DIV
D
V
V
1V/DIV
2
OUT1
OUT2
is the total power dissipated, V
BUS
ground plane on a double-sided 1oz copper
0V
0V
Figure 13. Power-Up Sequencing
–BAT) • I
D(REGS)
BAT
100μs/DIV
is the sum of power dissipated
+ P
D(REGS)
LTC3554
3554 F13
BAT
BUS
is the battery
is the supply
31
JA
) of
3554f