LTC3554-2_15 Datasheet, PDF (32/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators

LTC3554/LTC3554-1/
LTC3554-2/LTC3554-3
OPERATION
The power dissipated by a step-down switching regulator
can be estimated as follows:
P_D(SWx) = (B_OUTx • I_OUT) • (100 - Eff)/100
Where B_OUTx is the programmed output voltage, I_OUT
is the load current and Eff is the % efficiency which can
be measured or looked up on an efficiency table for the
programmed output voltage.
Thus the power dissipated by all regulators is:
P_D(REGS) = P_D(SW1) + P_D(SW2)
It is not necessary to perform any worst-case power dis-
sipation scenarios because the LTC3554 will automatically
reduce the charge current to maintain the die temperature
at approximately 110°C. However, the approximate ambi-
ent temperature at which the thermal feedback begins to
protect the IC is:
T_A = 110°C – P_D • q_JA
Example: Consider the LTC3554 operating from a wall
adapter with 5V (V_BUS) providing 400mA (I_BAT) to charge a
Li-Ion battery at 3.3V (BAT). Also assume P_D(REGS) = 0.3W,
so the total power dissipation is:
P_D = (5V – 3.3V) • 400mA + 0.3W = 0.98W
The ambient temperature above which the LTC3554 will be-
gin to reduce the 400mA charge current, is approximately:
T_A = 110°C – 0.98W • 70°C/W = 41.4°C
The LTC3554 can be used above 41.4°C, but the charge
current will be reduced below 400mA. The charge current
at a given ambient temperature can be approximated by:
P_D = (110°C – T_A) / θ_JA = (V_BUS – BAT) • I_BAT + P_D(REGS)
Thus:
I_BAT = [(110°C – T_A) / q_JA - P_D(REGS)]
(V_BUS – BAT)
Consider the above example with an ambient tem-
perature of 60°C. The charge current will be reduced to
approximately:
I_BAT = [(110°C - 60°C) / 70°C/W - 0.3W]/(5V – 3.3V)
I_BAT = (0.71W - 0.3W) / 1.7V = 241mA
Printed Circuit Board Layout
When laying out the printed circuit board, the following
list should be followed to ensure proper operation of the
LTC3554:
1. The Exposed Pad of the package (Pin 21) should connect
directly to a large ground plane to minimize thermal
and electrical impedance.
2. The trace to the step-down switching regulator input
supply pin (BVIN) and its decoupling capacitor should be
kept as short as possible. The GND side of this capacitor
should connect directly to the ground plane of the part.
This capacitor provides the AC current to the internal
power MOSFETs and their drivers. It is important to
minimize inductance from this capacitor to the pin of
the LTC3554. Connect BVIN to V_OUT through a short
low impedance trace.
3. The switching power traces connecting SW1 and SW2
to their respective inductors should be minimized to
reduce radiated EMI and parasitic coupling. Due to the
large voltage swing of the switching nodes, sensitive
nodes such as the feedback nodes (FB1 and FB2) should
be kept far away or shielded from the switching nodes
or poor performance could result.
4. Connections between the step-down switching regu-
lator inductors and their respective output capacitors
should be kept as short as possible. The GND side of
the output capacitors should connect directly to the
thermal ground plane of the part.
5. Keep the buck feedback pin traces (FB1 and FB2) as
short as possible. Minimize any parasitic capacitance
between the feedback traces and any switching node
(i.e., SW1, SW2 and logic signals). If necessary, shield
the feedback nodes with a GND trace.
6. Connections between the LTC3554 PowerPath pins
(V_BUS and V_OUT) and their respective decoupling ca-
pacitors should be kept as short as possible. The GND
side of these capacitors should connect directly to the
ground plane of the part.
32
3554123ff