DN464 - High Efficiency USB Power Management System Safely Charges Li-Ion/Polymer Batteries from Automotive Supplies

High Efficiency USB Power Management System Safely Charges
Li-Ion/Polymer Batteries from Automotive Supplies – Design Note 464
George H. Barbehenn
In this circuit, the LTC®4098 USB power manager/LiIon battery charger controls an LT3480 HV step-down
regulator. The LTC4098’s Bat-Track™ feature provides
a high efficiency, low power dissipation battery charger
from low and high voltages alike. The Bat-Track feature
controls an internal input current-limited switching regulator to regulate VOUT to approximately VBAT + 0.3V which
maximizes battery charger efficiency, and thus minimizes
power dissipation by operating the battery charger with
minimal headroom. Furthermore, the Bat-Track feature
reduces charge time by allowing a charge current greater
than the USB input current limit—the switching regulator
behaves like a transformer exchanging output voltage for
output current.
Introduction
Automotive power systems are unforgiving electronic
environments. Transients to 90V can occur when the
nominal voltage range is 10V to 15V (ISO7637), along with
battery reversal in some cases. It’s fairly straightforward
to build automotive electronics around this system, but
increasingly end users want to operate portable electronics, such as GPS systems or music/video players,
and to charge their Li-Ion batteries from the automotive
battery. To do so requires a compact, robust, efficient
and easy-to-design charging system.
Complete USB/Battery Charging Solution for Use in
Large Transient Environments
Figure 1 shows such a design. This complete PowerPath™
manager and battery charger system seamlessly charges
the Li-Ion battery from a wide ranging high voltage or
USB source.
M1
ZXMP10A18G
The LTC4098 can extend the Bat-Track concept to an
auxiliary regulator via the WALL and VC pins. When
sufficient voltage is present on WALL, Bat-Track takes
L, LT, LTC and LTM are registered trademarks and PowerPath and Bat-Track are
trademarks of Linear Technology Corporation. All other trademarks are the property of
their respective owners.
M2
ZXMP10A18G
HVIN
2
VIN
AUTOMOTIVE,
FIREWIRE,
ETC.
D1
MMBZ5240BLT1G
10V
C6
C1
4.7μF 68nF
R5
10k
R1
1k
R3
33k
4
VIN
R2
150k
5
RUN/SS
10
RT
R6
40.2k
GND
9
FB
BD SYNC
11
1
8
L1
3.3μH
20
USB
OVGATE
13
C2
10μF
0805
2
1
15-17
TO μC
8
TO μC
4
R7
100k
R8
T
100k
5
VBUS
18
VC WALL
19
VOUT
OVSENS
IDGATE
BAT
D0–D2
CHRG
M4
C5
10μF
0805
14
ACPR SW
OVGATE
C4
22μF
R12
100k
6
M3
ZXMN10A08E6
R4
10k
HVBUCK
R11
499k
LT3480
PG VC
7
C7
L2
0.47μF 10μH
BOOST
3
SW
12
10
11
LTC4098
SYSTEM
LOAD
NTCBIAS
NTC CLPROG PROG GND BATSENS
3
7
9, 21
6
C3
R9
R10
+
0.1μF
Li-Ion
2.94k
1k
0603
DN464 F01
Figure 1. LTC4098 USB Power Manager/Li-Ion Battery Charger Works with an LT®3480 HV Buck Regulator to Accept Power
from an Automotive Environment or Firewire System. Overvoltage Protection Protects Both ICs and Downstream Circuits
05/09/464
control of the auxiliary regulator’s output via the VC pin,
maintaining the regulator’s output at VBAT + 0.3V.
The LTC4098 also includes an overvoltage protection function—important in volatile supply voltage environments.
Overvoltage protection shuts off a protection N-channel
MOSFET (M2) when the voltage at the OVSENSE pin exceeds
approximately 6V. The upper limit of voltage protection is
limited only by the breakdown voltage of the MOSFET, and
by the current flowing into the OVSENS pin.
Overvoltage Protection Covers the Entire Battery
Charger/Power Manager System
The overvoltage protection function of the LTC4098 can
protect any part of the circuit. In Figure 1, the protection has
been extended to the LT3480 VIN input. The overvoltage
shutdown threshold has been set to 24V. This threshold
provides ample margin against destructive overvoltage
events without interfering with normal operation.
In Figure 1, M1 is a P-channel MOSFET that provides
reverse voltage protection, whereas M2 is the overvoltage
protection MOSFET, and M3 level-shifts the OVGATE
output of the LTC4098.
If the HVIN voltage is less than zero, the gate and source
voltages of both M1 and M2 are held at ground through R3,
R4, and R5, ensuring that they are off. If the HVIN voltage is
between 8V and approximately 24V, the gate of M3 is driven
high via the LTC4098’s OVGATE pin. This turns on M1 and
M2 by pulling their gates 7V to 10V below their sources via
M3, D1, R1 and R5. With M1 and M2 on current flows from
HVIN to VIN and the system operates normally.
If the HVIN input exceeds approximately 24V, the LTC4098
drives the gate of M3 to ground, which allows R5 to
reduce the VGS of M1 and M2 to zero, shutting them off
and disconnecting HVIN from VIN.
M1, M2 and M3 have a BVDSS of 100V, so that this circuit
can tolerate voltages of approximately –30V to 100V. It
will operate normally from 8V to approximately 24V. This
combination is ideal for the harsh automotive environment,
providing a robust, low cost and effective solution for Li-Ion
battery charging from an automotive power system.
Finally, setting the OVSENS resistor divider requires some
care. For an OVSENS voltage between approximately 2V
and 6V, VOVGATE = 1.9 • VOVSENS. OVSENS is clamped at
6V and the current into (or out of) OVSENS should not
exceed 10mA. The chosen resistor divider attenuates HVIN
by a factor of 4, so M3 has sufficient gate voltage to turn
on when HVIN exceeds approximately 8V. When HVIN
= 100V, the current into OVSENS is just 2.25mA—well
below the 10mA limit.
As shown in Figure 2, VIN is only present when HVIN
is in the 8V to 24V region. Figure 3 shows a close-up
centered on the load dump ramp. The ISO7637 test
ramp rises from 13.2V to 90V in 5ms. There is a 220μs
turn-off delay—OVGATE going low to the gates of M1
and M2—which results in an overshoot on VIN. The
maximum value of this overshoot is 3.5V (V VIN(MAX) ≈
27.5V). The magnitude of this overshoot can be calculated
for different ramp rates, such that
VOVERSHOOT = ΔV/Δt • tDELAY
where ΔV = (90V – 13.6V), Δt = 5ms, and tDELAY = 220μs,
so, VOVERSHOOT = 3.36V.
If less delay, and thus less overshoot, is desired, an active
turn-off circuit can reduce the delay from OVGATE to the
gates of M1 and M2 to a few microseconds.
Conclusion
The LT3480 high voltage step-down regulator and LTC4098
Li-Ion/Polymer battery charger, combined with a few
external components, produce a robust high performance
Li-Ion charger suitable for portable electronics plugged into
an automotive power source and maintain compatibility
with USB power. The circuit provides all the functionality
that customers expect, along with voltage protection from
battery reversal and load dump transients.
TRACE 1 = OVGATE
10V/DIV
TRACE 1 = OVGATE
10V/DIV
TRACE 2 = HVBUCK
5V/DIV
TRACE 2 = HVBUCK
5V/DIV
TRACE 3 = VIN
20V/DIV
TRACE 3 = VIN
20V/DIV
TRACE 4 = HVIN
50V/DIV
TRACE 4 = HVIN
50V/DIV
200ms/DIV
DN464 F02
Figure 2. Overvoltage Protection Through
Input Transients per ISO7637 Standards
Data Sheet Download
www.linear.com
2ms/DIV
DN464 F03
Figure 3. Closeup of Figure 2 Waveforms
Showing Overshoot on HVIN
For applications help,
call (978) 656-4700, Ext. 3752
Linear Technology Corporation
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