LTC3576-1_15 Datasheet, PDF (36/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go Triple Step-Down DC/DCs

LTC3576/LTC3576-1
APPLICATIONS INFORMATION
USB/WALL
ADAPTER
MN1
R1
V_BUS
C1 _LTC3576/
LTC3576-1
OVGATE
OVSENS
3576 F06
Figure 6. Overvoltage Protection
USB/WALL
ADAPTER
MP1
MN1
D1
C1
R1
R2
V_BUS POSITIVE PROTECTION UP TO BVDSS OF MN1
V_BUS NEGATIVE PROTECTION UP TO BVDSS OF MP1
V_BUS
LTC3576/
LTC3576-1
OVGATE
OVSENS
3576 F08
Figure 8. Dual Polarity Voltage Protection
M1
V1
WALL
V2
D2
D1 ^M2
OVGATE
LTC3576/
LTC3576-1
V_BUS
C1
GND
R1
OVSENS
3576 F07
Figure 7. Dual-Input Overvoltage Protection
It is possible to protect both V_BUS and WALL from
overvoltage damage with several additional components,
as shown in Figure 7. Schottky diodes D1 and D2 pass the
larger of V1 and V2 to R1 and OVSENS. If either V1 or V2
exceeds 6V plus V_F (Schottky), OVGATE will be pulled to
GND and both the WALL and USB inputs will be protected.
Each input is protected up to the drain-source breakdown,
BVDSS, of MN1 and MN2. R1 must also be rated for the
power dissipated during maximum overvoltage.
Reverse Voltage Protection
The LTC3576/LTC3576-1 can also be easily protected
against the application of reverse voltages, as shown in
Figure 8. D1 and R1 are necessary to limit the maximum
V_GS seen by MP1 during positive overvoltage events. D1’s
breakdown voltage must be safely below MP1’s BVGS. The
circuit shown in Figure 8 offers forward voltage protection
up to MN1’s BVDSS and reverse voltage protection up to
MP1’s BVDSS.
Battery Charger Over Programming
The USB high power specification allows for up to 2.5W
to be drawn from the USB port. The LTC3576/LTC3576-1’s
bidirectional switching regulator in step-down mode
36
transforms the voltage at V_BUS to a voltage just above
the level at BAT, while limiting power to less than the
amount programmed at CLPROG. The charger should be
programmed (with the PROG pin) to deliver the maximum
safe charging current without regard to the USB specifi-
cations. If there is insufficient current available to charge
the battery at the programmed rate, it will reduce charge
current until the system load on V_OUT is satisfied and the
V_BUS current limit is satisfied. Programming the charger
for more current than is available will not cause the aver-
age input current limit to be violated. It will merely allow
the battery charger to make use of all available power to
charge the battery as quickly as possible, and with minimal
dissipation within the charger.
Battery Charger Stability Considerations
The LTC3576/LTC3576-1’s battery charger contains both a
constant-voltage and a constant-current control loop. The
constant-voltage loop is stable without any compensation
when a battery is connected with low impedance leads.
Excessive lead length, however, may add enough series
inductance to require a bypass capacitor of at least 1μF
from BAT to GND.
High value, low ESR MLCCs reduce the constant-voltage
loop phase margin, possibly resulting in instability. Up
to 22μF may be used in parallel with a battery, but larger
capacitors should be decoupled with 0.2Ω to 1Ω of series
resistance.
Furthermore, a 100μF MLCC in series with a 0.3Ω resistor
from BAT to GND is required to prevent oscillation when
the battery is disconnected.
In constant-current mode, the PROG pin is in the feed-
back loop rather than the battery voltage. Because of the
additional pole created by any PROG pin capacitance,
3576fb