AN-9720 [FAIRCHILD]

Power Path Implementation Tradeoffs; 电源路径实施的权衡


型号：	AN-9720
厂家：	FAIRCHILD SEMICONDUCTOR
描述：	Power Path Implementation Tradeoffs 电源路径实施的权衡
文件：	总7页 (文件大小：230K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

www.fairchildsemi.com

AN-9720

Power Path Implementation Tradeoffs, Featuring the

FAN5400 Family of PWM Battery Chargers

Overview

Many battery-powered systems, such as smart phones, must be

fully functional and their electronics up and running shortly

after the user plugs a charger into the phone. The system

electronics need to be functional regardless of the state of the

battery, even if the battery is fully discharged or absent. The

FAN5400 family charger IC allows power to be delivered to

the system when the charger is plugged in, whether the battery

is absent or present, and allows the system to power up and be

functional quickly after the charger is plugged in. Other

approaches, such as power path, add additional impedance in

series with the battery.

While this implementation meets the criteria for power path,

an ideal diode is never truly ideal. For example, the internal

ideal diode of one such IC is actually a PMOS with a typical

value of 180mΩ. This means that there is a permanent 180mΩ

power dissipating series element between battery and system

load that creates significant additional power loss during

periods of high-current drain from the battery, such as GSM

pulses. A parallel diode PMOS switch can reduce this

resistance; however, this increases the solution size and cost.

The implementation in Figure 1 differs from the FAN540X

approach in the block diagram in Figure 2. Although on the

surface it may seem like FAN5400 does not have power path

functionality; it serves almost all the same needs and, in

addition, can provide the benefit of having no power

dissipating series element between system load and battery.

This application note describes how the FAN5400 family of

battery chargers distributes power between the system and

battery to achieve similar results of power path, often without

the overhead and power loss of an additional switch element.

The block diagram in Figure 1 is a typical implementation of

power path using an “ideal” diode. Current flow is indicated

by the arrows, which reveal that the “ideal” diode (whether

internal or external) helps to steer current appropriately.

Figure 1. Typical Implementation of Power Path using an “Ideal” Diode

Rev. 1.0.0 • 12/23/10

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AN-9720

APPLICATION NOTE

VREG

C_REG

1.8V / PMID REG

PMID

C_MID

4.7

VBUS

C_BUS

CHARGE

PUMP

Q1A

Q1B

POWER

OUTPUT

STAGE

PWM

MODULATOR

R_SENSE

I_IN

CONTROL

VBUS

OVP

C_OUT

0.1

PGND

ISNS

VCC

CSIN

VBAT

DAC

VREF

Battery

C_BAT

SDA

SCL

PMID

SYSTEM

LOAD

I2C

INTERFACE

STAT

30mA

OSC

DISABLE

LOGIC

AND

OTG/USB#

CONTROL

Figure 2. FAN5400 Block Diagram

Example 1: 1500mAh Battery (1C Maximum Charge

Current Capability of Battery is 1500mA), Input

Power Source USB 2.0, 5V 500mA

System and Battery Power Sharing

Power sharing between the system and the battery means that

power can be steered or prioritized to go to the system in the

case that the input power is not sufficient to power both the

system and charge the battery.

Scenario A)

Partially charged battery at 3.6V and

system load turns on at 400mA

Before the system load turns on, the charger is already in CC

mode. Because the input power source is 5V 500mA and the

battery is at 3.6V, there is ~632mA of current available to

charge the battery. This is computed by accounting for the

charger conversion efficiency and also the output current

multiplication factor achieved when bucking down a voltage:

The typical configuration for the FAN5400, as shown in

Figure 3, is one where the system is connected in parallel to

the battery. The way in which this configuration can steer

power similar to power path is sometimes confusing, so

scenarios based on real-world battery capacity and input

power numbers are provided below.

I_OUTMAX= V_IN/V_OUT∗ I_IN∗ Efficiency

(1)

Using the values in this example reveals that

5V/3.6V•500mA•91%=632mA. The 91% efficiency data

point can be found in Figure 4.

94%

92%

90%

88%

Figure 3. Typical Application Circuit, System Parallel

with Battery

86%

84%

4.5VBUS

5.0VBUS

5.5VBUS

2.5

2.7

2.9

3.1

3.3

3.5

3.7

3.9

4.1

4.3

Battery Voltage, VBAT (V)

Figure 4.

FAN5400 Conversion Efficiency vs.

Battery Voltage vs. V_BUSVoltage

www.fairchildsemi.com

Rev. 1.0.0 • 12/23/10

AN-9720

APPLICATION NOTE

Once the system load turns on, 400mA is diverted to the

system and only 232mA is left to charge the battery. This is

the equivalent of power steering; the charger is prioritizing the

system over the battery. Once the system load turns off, the

full 632mA once again flows to the battery. The benefit of

FAN5400, as configured in Figure 3, is that there is no power-

dissipating series element between the system and load.

Scenario D)

load turns on at 2000mA

Battery is fully charged at 4.2V and system

Before the system load turns on, the charger is off. When the

system load turns on, the power first comes from the battery

and almost immediately the battery charger turns on and goes

into CC Mode. This is because Li-Ion batteries typically have

an output impedance of 150mΩ, which almost instantly forces

V_BAT< V_OREG- V_RCH. Similar to Scenario C, the charger

attempts to charge the battery at 575mA. (Actually slightly

higher than 575mA because battery voltage is lower in this

case, as compared to Scenario C, and the multiplication factor

is slightly higher. For the sake of this exercise, it is

inconsequential.) The charger attempts to charge; however,

because the system load is 2000mA, the 575mA flows to the

load and the remaining system load of 1425mA comes from

the battery.

Scenario B)

Partially charged battery at 3.6V and

system load turns on at 2000mA

Before the system load turns on, the charger is already in CC

mode and using all input power to charge the battery at

632mA, similar to Scenario A. When the system load turns on,

632mA is diverted to the system and the remaining system

load of 1368mA comes from the battery.

This is the equivalent of power steering; the charger is

prioritizing the system over the battery. Once the system load

turns off, the full 632mA once again flows to the battery.

Again, the circuit in Figure 3 has an advantage of no

dissipating element between the system and load.

This is the equivalent of power steering; the charger is

prioritizing the system over the battery. Once the system load

turns off, the full 575mA flows to the battery until the battery

enters CV Mode, at which point, the charge current decreases.

Again, the circuit in Figure 3 has an advantage of no

dissipating element between system and load.

Scenario C)

Battery is fully charged at 4.2V and system

load turns on at 400mA

Example 2: Assume a 700mAh Battery (1C

Maximum Charge Current Capability of Battery

700mA), Input Power Source AC/DC Adaptor 5V

900mA or USB 3.0, 5V, 900mA

Before the system load turns on, the charger is off. When the

load turns on, all the system power first comes from the

battery. As soon as V_BAT< V_OREG- V_RCH, the charger turns on.

V_RCHis the recharge threshold and has a value of 120mV.

Because the input power source is 5V 500mA, the maximum

available current the charger can provide is computed as

5V/4V•500mA•92%=575mA (battery is assumed to have to

4V for the sake of this exercise). The charger turns on,

attempting to charge the battery at 575mA. However, because

the system load is still present, only 575mA-400mA=175mA

is flowing into the battery.

Scenario A)

Partially charged battery at 3.6V and

system load turns on at 400mA

Before the system load turns on, the charger is in CC Mode.

Because the battery is at 3.6V, there is 5V/3.6V•900mA•91%

=1138mA of current available to charge the battery. However,

the battery is limited to a maximum 1C charge current of

700mA and, therefore, the charger is set to charge at 700mA.

What is unique in Example 2, in comparison to Example 1, is

that the input power supply can supply more power than the

battery is able to accept in 1C scenario. When the system load

turns on, 400mA is diverted to the system and only 300mA is

left to charge the battery.

This is the equivalent of power steering; the charger is

prioritizing the system over the battery. Once the system load

turns off, the full 575mA flows to the battery until the battery

enters CV Mode; at which point, the charge current decreases.

Again, the circuit in Figure 3 has an advantage of no

dissipating element between system and load.

Some designers might object because the input power supply

is not being used to the full extent that the combination of the

battery and system load can accept. One solution is to connect

the system load to CSIN, as shown in Figure 5.

Figure 5. Application Circuit with System Load Connected to CSIN

Rev. 1.0.0 • 12/23/10

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AN-9720

APPLICATION NOTE

Connecting the system load to CSIN allows the AC/DC

adaptor or USB source to supply current up to maximum

power level even if that is higher than the battery’s 1C level.

In this configuration, before the load turns on, the battery is

charging at its 1C maximum charge capability of 700mA.

When the 400mA system load turns on, the entire 400mA

system load is supplied by the charger and the battery

continues to charge at 700mA.

Scenario D)

load turns on at 2000mA

Battery is fully charged at 4.2V and system

Before the system load turns on, the charger is off. When the

system load turns on; if the configuration from Figure 3 is

used, the power comes first from the battery and almost

immediately the battery charger turns on and goes into CC

Mode. This is because Li-Ion batteries typically have an

output impedance of 150mΩ, which almost instantly forces

V_BAT< V_OREG- V_RCH. Similar to Scenario C, the charger

attempts to charge the battery at 700mA. However, because

the system load is 2000mA, the 700mA flows from the

charger to the system load and the remaining system load of

1300mA comes from the battery.

One drawback to this configuration is that there is a power

dissipating 68mΩ series element always in the path between

the battery and the system load. This constant power

dissipating element between battery and system load is similar

to the one found in the power path implementation in Figure 1.

However, the 68mΩ in FAN5400 is substantially less than the

180mΩ found in some products with power path.

If the configuration in Figure 5 is used; when the load turns

on, 1035mA flows from the charger to the load and the

remaining 965mA is supplied by the battery. Once the system

load turns off, 700mA flows to the battery until the battery

enters CV Mode; at which point, the charge current begins to

decrease. Again, the tradeoff is the 68Ω-series element

between the battery and system load.

Scenario B)

Partially charged battery at 3.6V and

system load turns on at 2000mA

Before the system load turns on, the charger is in CC Mode

and charging the battery at 700mA, similar to Scenario A.

When the system load turns on, if the configuration from

Figure 3 is used, 700mA is supplied to the system load from

the charger and the remaining 1300mA by the battery.

Charge Termination when System Load is

Connected to CSIN

If the configuration from Figure 5 is used instead, 1138mA is

supplied to the system load from the charger and the

remaining 862mA is supplied by the battery.

When the system load is connected to CSIN as it is in Figure

5; if the system load is larger than the power that can be

supplied from the USB port with the I_BUSlimit (e.g.

500mA), the battery current is reduced below the

termination threshold. Normally, when the battery current is

reduced below the termination threshold, charge is

terminated. However, if the charger input is in current limit,

the FAN540X does not allow charge termination. Charge

termination occurs only if I_BATis less than I_TERMand the

charger input is not in current limit for at least 32ms. This

prevents false termination from system loading.

Both configurations are the equivalent of power steering; but

in the configuration of Figure 5, all the input power is being

used. The tradeoff is the 68mΩ-series element between battery

and system load. Once the system load turns off, 700mA flows

to the battery in both configurations.

Scenario C)

Battery is fully charged at 4.2V and system

load turns on at 400mA

After charge termination occurs, the charger stops running and

system power is drawn from the battery. Charge termination

can be disabled by disabling the TE bit through I²C. Charge

termination may, however, be desired if the phone or data card

is in a low-power mode and a drain on the USB port is not

desired. In that case, recharge occurs automatically when the

battery voltage drops V_RCH(120mV) below V_OREGor if it is

desired to turn on before 120mV, the system can manually

turn on the charger as soon a drop in V_BATis sensed.

Before the system load turns on, the charger is off. When the

load turns on, the system power first comes from the battery.

As soon as V_BAT< V_OREG- V_RCH, the charger turns on. V_RCHis

the recharge threshold and has a value of 120mV. Because the

input power source is 5V 900mA, the maximum available

current the charger can provide is 5V/4V•900mA•92%

=1035mA (assume the battery has dropped to 4V for the sake

of this exercise). The charger turns on, attempting to charge

the battery at 700mA. However, because the system load is

still present, if the configuration in Figure 3 is used; only

300mA is left to charge the battery.

Powering the System with No Battery

If the configuration in Figure 5 is used; when the load turns

on, 635mA flows to the battery and 400mA is supplied from

the charger to the system load. This makes sense because there

is a total of 1035mA charger output current. Once the system

load turns off, the full 700mA flows to the battery until the

battery enters CV Mode; at which point, the charge current

decreases. The tradeoff is the 68mΩ-series element between

the battery and system load.

The FAN5402 and FAN5405 continues charging after VBUS

POR with the default parameters, regulating the V_BATline to

3.54V until the host processor issues commands or the 15-

minute timer expires. In this way, the FAN5402/05 can start

the system without a battery.

The FAN5400 family’s soft-start function can interfere with

the system supply with battery absent. The soft-start activates

Rev. 1.0.0 • 12/23/10

www.fairchildsemi.com

AN-9720

APPLICATION NOTE

whenever V_OREG, I_INLIM, or I_OCHARGEare set from a lower to

higher value. During soft-start, the I_INlimit drops to 100mA

for about 1ms, unless I_INLIMis set to 11 (“no limit”). This

could cause the system processor to fail to start. To avoid this

behavior, use the following sequence.

To understand how quickly the battery voltage rises, it is

necessary to examine the internal operation of the battery

pack. Inside every battery pack is a protection IC, as shown in

Figure 6, that features two back-to-back MOSFETs and an

analog control circuit to prevent over-charging and over-

discharging by monitoring the cell voltage and discharge

current. The protection circuit is also referred to as “secondary

protection” since the charging system must also ensure that the

battery is not over-charged. The protection circuit provides a

back-up safety circuit where overcharging is concerned.

1. Set the OTG pin HIGH. When VBUS is plugged in,

I_INLIMis set to 500mA until the system processor powers

up and can set parameters through I²C.

2. Program the Safety Register

3. Set I_INLIMto 11 (no limit).

4. Set OREG to the desired value (typically 4.18)

5. Reset IOLEVEL bit, then set IOCHARGE.

PROTECTION

CIRCUIT

CELL

6. Set I_INLIMto 500mA if a USB source is connected or any

other level that is preferred.

ESR

–

During the initial system startup, while the charger IC is being

programmed, the system current is limited to 340mA for 1ms

during steps 4 and 5. This is the value of the soft-start I_CHARGE

current used when I_INLIMis set to no limit.

CONTROL

If the system powers up without a battery present, the CV bit

should be set. When a battery is inserted, the CV bit clears.

Figure 6. Li-Ion Battery Pack

Powering a System when the

Battery is Deeply Discharged

When the battery voltage is lower than the voltage required

to power the system load, the power routing implementation

(Error! Reference source not found. and Figure 5) cannot

bring up the system.

Q2 in the protection circuit opens if the cell is deeply

discharged (V_CELL< 2.7V). Charging is therefore still

possible by driving current into the pack through Q2’s body

diode. Refer to Figure 7 for the different states.

In contrast, power path, as implemented in Figure 1, can still

power the system even if the battery voltage is substantially

lower than the system load needs to operate. This is the

primary advantage of the circuit in Figure 1 as compared to

FAN5400. However, it is important to realize that the slope

of the curve during this phase of the charge cycle is

incredibly steep, which means that the battery voltage is

brought up to the minimum system load requirement within

a matter of seconds.

Rev. 1.0.0 • 12/23/10

www.fairchildsemi.com

AN-9720

APPLICATION NOTE

Table 1. Internal Batter Protections Scheme

Condition

Over-Charge

Detection

V_CELL>4.25

Response

Open Q1

Open Q2

Reset

V_CELL>4.10

V_CELL>2.95

V_CELL≈ V_BAT

Over-Discharge

Over I_DISCHARGE

V_CELL<2.7

V_CELL– V_BAT>X

In the extreme case that the battery is deeply discharged, the

discharge protection switch (Q2) is open. When the charger is

plugged in the USB port; if OTG = 0, the trickle charge of

30mA charges the system capacitor to 2V in 20ms time (refer

to Table 2). The charger continues to run in “auto” mode until

the applications processor wakes up at 3.4V in about 15-

seconds time. The applications processor then enumerates

with USB and sets I_BUS= 500mA and I_CHARGE= 700mA.

Table 2. Bringing Battery Out of Deep Discharge

Condition

V_BAT

I_BAT

Comment

0 to

2.0

V_BAT=EMF+0.7

Charger is charging system cap.

Linear Charge (20ms)

30mA

V_BAT=(EMF=+0.7+.1•ESR)

100mA Some charge going to VCELL

t_15MIN(default) PWM Charge

with I_BUSset by OTG Pin (15s)

2.0 to

3.4

Apps processor wakes up at V_BAT≈3.4V

Higher charge current set by processor. This causes V_CELLto rise, which

3.5+ 700mA causes Q2 to close at point A in Figure 7. V_BATdrops to about 2.9 and

charging continues from there.

High-Current PWM Charge

1s to Q2 Close

The charge cycle behavior is shown graphically in Figure 7.

The time it takes to charge a typical battery to a stable 3.4V

is 40 seconds. This process is explained below.

4.00

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

3.50

Processor wakes up when

V_BAT> 3.4V, which occurs

3.00

2.50

2.00

1.50

1.00

0.50

about 15s after V_BUSis

plugged in.

VBAT

Battery Current (A)

Q2 closes about 1s after

the processor programs

the IC for higher charge

current. This causes V_BAT

to drop (no more diode in

series with V_BAT).

100

120

140

Time (seconds)

Figure 7. Charge Characteristics for Deeply Discharged and Dead Batteries

Rev. 1.0.0 • 12/23/10

www.fairchildsemi.com

AN-9720

APPLICATION NOTE

Conclusion

Although there are situations where the FAN540X partial

power path in Figure 2 cannot immediately power the system

load when the battery is very low or deeply discharged, this

timeframe for a typical cell phone battery is only 40 seconds.

It is important to weigh this against the benefits offered by the

FAN540X. This benefit was discussed in detail, showing that

the FAN5400, configured as in Figure 3, does not have a

power-dissipating series element between battery and system

load and provides the two more critical aspects of dynamic

power routing… power sharing between the system and the

battery and powering the system with no battery.

Related Resources

FAN5400 Family — USB-Compliant Single-Cell Li-Ion Switching Charger with USB-OTG Boost Regulator

AN-1721 — Li-Ion Battery Charging Basics, Featuring the FAN5400 / FAN5420 Family of PWM Battery Chargers

DISCLAIMER

FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS

HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE

APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS

PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.

LIFE SUPPORT POLICY

FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS

WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION.

As used herein:

1. Life support devices or systems are devices or systems

which, (a) are intended for surgical implant into the body, or

(b) support or sustain life, or (c) whose failure to perform

when properly used in accordance with instructions for use

provided in the labeling, can be reasonably expected to

result in significant injury to the user.

2. A critical component is any component of a life support

device or system whose failure to perform can be

reasonably expected to cause the failure of the life support

device or system, or to affect its safety or effectiveness.

Rev. 1.0.0 • 12/23/10

www.fairchildsemi.com

AN-9720 [FAIRCHILD]

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