LTC3553-2_15 Datasheet, PDF (30/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger, Always-On LDO and Buck Regulator

LTC3553-2
OPERATION
Power-Up Sequencing
Figure 12 shows the actual power-up sequencing of the
LTC3553-2. The regulators are both initially disabled
(0V). Starting in hard reset state, if the pushbutton has
been applied (ON low) for 400ms, the LDO is enabled.
The LDO slews up and enters regulation. The actual slew
rate is controlled by the soft start function of the LDO in
conjunction with output capacitance and load (see the LDO
Regulator Operation section for more information). When
the LDO is within about 8% of final regulation, the buck
is enabled and slews up into regulation. 1.8ms after the
buck is within 8% of final regulation, the PGOOD output
will go high impedance. The regulators in Figure 12 are
slewing up with nominal output capacitors and no-load.
Adding a load or increasing output capacitance on any of
the outputs will reduce the slew rate and lengthen the time
it takes the regulator to achieve 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 LTC3553-2 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 LTC3553-2 has a thermal resistance (θ_JA) 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 LTC3553-2 to reduce charge
current due to the thermal protection feedback can be
approximated by considering the power dissipated in the
BUCK OUTPUT
0.5V/DIV
0V
LDO OUTPUT
1V/DIV
0V
100μs/DIV
35532 F12
Figure 12. Power-Up Sequencing, Front Page
Application Circuit
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