LTC4101EG Просмотр технического описания (PDF) - Linear Technology

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LTC4101EG

Smart Battery Charger Controller

Linear Technology 

LTC4101

APPLICATIONS INFORMATION

The following equation shows the minimum COUT (±20%

tolerance) capacitance values for stability when used with

the compensation shown in the typical application on the

back page.

COUT(MIN) = 120/L1

The use of aluminum electrolytic for C1, located at the

AC adapter input terminal, is helpful in reducing ringing

during the hot-plug event. Refer to Application Note 88

for more information.

In the 4A lithium battery charger (typical application on

back page), the input capacitor (C2) is assumed to absorb

all input switching ripple current in the converter, so it

must have adequate ripple current rating. Worst-case RMS

ripple current will be equal to one half of output charging

current. C2 is recommended to be equal to or greater than

C4 (output capacitor) in capacitance value.

The output capacitor (C4) is also assumed to absorb

output switching current ripple. The general formula for

capacitor current is:

IRMS

=

0.29(VBAT )⎛⎝⎜ 1–

(L1)(f)

VBAT

VDCIN

⎞

⎠⎟

For example, VDCIN = 12V, VBAT = 4.2V, L1 = 10μH, and

f = 300kHz, IRMS = 0.26A.

EMI considerations usually make it desirable to minimize

ripple current in the battery leads, and beads or induc-

tors may be added to increase battery impedance at the

300kHz switching frequency. Switching ripple current splits

between the battery and the output capacitor depending

on the ESR of the output capacitor and the battery imped-

ance. If the ESR of C3 is 0.2Ω and the battery impedance

is raised to 4Ω with a bead or inductor, only 5% of the

current ripple will flow in the battery.

Protecting SMBus Inputs

The SMBus inputs, SCL and SDA, are exposed to uncon-

trolled transient signals whenever a battery is connected

to the system. If the battery contains a static charge, the

SMBus inputs are subjected to transients which can cause

damage after repeated exposure. Also, if the battery’s posi-

26

tive terminal makes contact to the connector before the

negative terminal, the SMBus inputs can be forced below

ground with the full battery potential, causing a potential

for latch-up in any of the devices connected to the SMBus

inputs. Therefore it is good design practice to protect the

SMBus inputs as shown in Figure 10.

VDD

CONNECTOR

TO BATTERY

TO SYSTEM

4101 F13

Figure 10. Recommended SMBus Transient Protection

SafetySignal (Thermistor) Value

The SafetySignal (typical application on back page), is a

multifunction signal that communicates three pieces of

information in order of importance:

1) Presence of the smart battery

2) The maximum time duration of the wake-up charge

allowed.

3) An optional and redundant temperature measurement

system.

The value of the resistance to ground communicates all

this information. The resistance ranges and what it does

is covered by the SBS Smart Battery Charger standard in

Section 6. Basically if you have a battery chemistry, such

as Li-ion, that cannot safely withstand an infinite dura-

tion wake-up charge, the SafetySignal resistance value

needs to be less than 425Ω. The popular value to use is

a fixed 300Ω resistor. Otherwise the resistance value is

10k which is normally expected to be done using a 10k

NTC resistor.

PCB Layout Considerations

For maximum efficiency, the switch node rise and fall times

should be minimized. To prevent magnetic and electrical

field radiation and high frequency resonant problems,

proper layout of the components connected to the IC is

4101fa

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