Simplify Battery Charging from the USB – Design Note 320
John Shannon
Introduction
The USB is fast becoming the most popular way to attach peripherals to personal computers. An important
feature of the USB standard is that it allows for power
transmission over the USB cable, making it possible for
a PDA or other handheld device to charge its battery
while it is connected to a host PC.
The charger is based on the LTC4053—a standalone
timer-terminated constant current/constant voltage
linear charger. The LTC4053 features a thermal loop
which maximizes the charging rate under all conditions by regulating the maximum die temperature. Less
sophisticated linear chargers must run lower charging
rates or risk overheating, thus increasing charge times.
The USB specification defines load current limits that
USB peripherals can draw from the USB. Three operating modes are allowed:
Operation of the LTC4053 is simple. The charge current
is set by a resistor on the PROG pin, the termination
timer period is set by a capacitor on the timer pin and
float voltage is preset at 4.2V.
• Suspend mode, where the USB peripheral draws
less than 500μA from the USB cable
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
• Low power mode, where the current consumption
can be up to 100mA
LTC4053
1
USB
INPUT
• High power mode, where the current is allowed to
be up to 500mA
USB
5V
2
Q3
IRLML6401
3
+
C2
4.7μF
C1
0.1μF
R9
10k
4
5
3
0.1μF
4
5
ACPR
BAT
VCC
FAULT SHDN
TIMER PROG
NTC
GND
10
9
4:45&.-0"%
8
7
4.3k
+
6
16.5k
4)65%08/
Li-Ion
'0364#
#"55&3: $0.1-*"/5
4641&/%.0%&
USB
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FEATURES:
MODE
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Figure 1 shows a minimum component solution to the
USB battery charger problem. In this case, only five
components including the LTC ®4053, are required to
achieve a fully compliant USB charger.
D5
ZHCS1000
1
+
4.7μF
The USB specification also states that USB peripherals
are not allowed to backfeed into the USB cable.
5V
WALL
ADAPTER
2
CHRG
Figure 1. Minimum Component Count USB
Compliant Li-Ion Charger
LTC4053
CHRG
VCC
ACPR
BAT
FAULT SHDN
TIMER PROG
GND
NTC
10
9
SYSTEM
LOAD
8
7
OPTIONAL
6
R8
1.50k
R1
16.5k
R7
RT1
4.32k 10k
+
Li-Ion
BATTERY
USB MODE
100mA 500mA
Q2
Q1
2N7002 2N7002
DN320 F02
Figure 2. Dual Input Charger: 1A Fast Charging from Wall Adapter or 100mA/500mA from USB
09/03/320_conv
Faster Charging with System in Full Operation
Both of these simple circuits draw system power directly
from the battery. This makes for a simple solution but
also reduces the total available power from the USB
port. The USB port is capable of 2.5W (5V • 500mA).
Placing the device system bus on the battery directly
(normally 3.6V) only allows the system to extract 1.8W
(3.6V • 500mA) from the USB cable before the 500mA
limit is reached and the battery must supply any excess
power. The lower the battery voltage, the less power is
available to the system load. The solution is to power
the system load directly from the USB, measure the
system current consumption and back off the charger
current as required to maintain USB compliance.
Charging from USB or a Wall Adapter
USB charge current is limited to 500mA, but the LTC4053
is capable of higher output currents. This capability can
be exploited by adding some additional components
around the LTC4053 to allow for a higher charge rate
from an alternate power source such as a wall adapter.
The circuit shown in Figure 2 requires only 12 components, is compliant with the USB specification and
provides high rate charging from an adapter. The NTC
feature of the LTC4053 is also implemented so that
charging is inhibited if the battery is too hot or too cold.
Two things must happen to add high rate adapter-based
charging. The first is that the USB input must be isolated
from the adapter to prevent adapter power feeding into
the USB. The second is that the higher charging rate
must be selectable by changing the resistive load on
the PROG pin.
Figure 3 shows a compact seven component solution
using the LTC4410 USB power manager. The system
load is supported directly by the USB when the USB is
present. This allows a maximum of 2.5W for the system
load. The current flowing from the USB port is measured
and the battery charging current is reduced in order to
maintain USB compliance. When USB power is absent,
the load is supported by the battery through the switch
Q1. Compared to the solution shown in Figure 2, this
solution more fully exploits the power available at the
USB port, thus resulting in faster charging.
A P-channel MOSFET, Q3, is used to select between
USB or adapter power. Q3 is on if USB power is present
and the adapter is absent since its gate is at ground
(through R9) and its source is at 5V. As the wall adapter
power comes up, the gate of Q3 is pulled high and Q3
turns off. A 1A charging current flows from the adapter
through diode D5. The 1A charge rate is programmed
by turning on Q2 when the adapter is present which
alters the resistive load on the PROG pin.
LTC4410
1
2
3
VIN
VOUT
GND
CHP
MODE USBP
6
SYSTEM POWER
SUPPLY
5
4
100mA 500mA
LTC4053
1
USB
INPUT
2
+
3
4.7μF
0.1μF
4
5
CHRG
VCC
ACPR
BAT
FAULT SHDN
TIMER PROG
GND
NTC
10
9
8
Q1
OFF ON
7
6
3.40k
10k
NTC
+
Li-Ion
BATTERY
DN320 F03
Figure 3. USB Compliant Charger Based on the LTC4410
Fully Utilizes the Power Available from the USB Input
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