LTC4101
25
4101fa
APPLICATIONS INFORMATION
c) I
ACCEL
is the current used by the SMBus accelerators.
This directly depends on the SMBus frequency, duty
cycle of messages sent on the SMBus and how long
it takes to drive the SMBus to V
DD
.
I
ACCEL
= I
PULL-UP
• 2 • SMBus Frequency •
Message Duty Cycle•V
DD
/2.25V•Rise Time
Complete Examples
1) Battery thermistor = 400Ω, V
DD
= 5.0V
Battery mode (DC is off), SMBus activity is 10kHz
and a 2% SMBus duty cycle, which represents a
suspended or sleep condition of a notebook.
I
TOTAL
= I
RUN
+ I
THRM(OFF)
+ I
ACCEL
= 121.9µA + 5.26µA + 2.44µA = 130µA
Battery mode and a 10% SMBus duty cycle, which
represents an active notebook at idle.
I
TOTAL
= I
RUN
+ I
THRM(OFF)
+ I
ACCEL
= 189.5µA + 5.26µA + 12.2µA = 207µA
DCIN = ON and a 20% SMBus duty cycle which
represents an active notebook charging.
I
TOTAL
= I
RUN
+ I
THRM(ON)
+ I
ACCEL
= 274µA + 215.6µA + 24.4µA = 514µA
2) Battery thermistor = 10k, V
DD
= 5.0V
Battery mode (DC is off), SMBus activity is 10kHz
and a 2% SMBus duty cycle:
I
TOTAL
= I
RUN
+ I
THRM(OFF)
+ I
ACCEL
= 121.9µA + 2.14µA + 2.44µA = 126µA
Battery mode and a 10% SMBus duty cycle:
I
TOTAL
= I
RUN
+ I
THRM(OFF)
+ I
ACCEL
= 189.5µA + 2.14µA + 12.2µA = 204µA
DCIN = ON and a 20% SMBus duty cycle:
I
TOTAL
= I
RUN
+ I
THRM(ON)
+ I
ACCEL
= 274µA + 37.7µA + 24.4µA = 336µA
Soft-Start and Undervoltage Lockout
The LTC4101 is soft-started by the 0.12µF capacitor on
the I
TH
pin. On start-up, I
TH
pin voltage will rise quickly
to 0.5V, then ramp up at a rate set by the internal 30µA
pull-up current and the external capacitor. Battery charging
current starts ramping up when I
TH
voltage reaches 0.8V
and full current is achieved with I
TH
at 2V. With a 0.12µF
capacitor, time to reach full charge current is about 2ms and
it is assumed that input voltage to the charger will reach
full value in less than 2ms. The capacitor can be increased
up to 1µF if longer input start-up times are needed.
In any switching regulator, conventional timer-based
soft-starting can be defeated if the input voltage rises much
slower than the time out period. This happens because
the switching regulators in the battery charger and the
computer power supply are typically supplying a fi xed
amount of power to the load. If input voltage comes up
slowly compared to the soft-start time, the regulators will
try to deliver full power to the load when the input voltage
is still well below its fi nal value. If the adapter is current
limited, it cannot deliver full power at reduced output
voltages and the possibility exists for a quasi “latch” state
where the adapter output stays in a current limited state at
reduced output voltage. For instance, if maximum charger
plus computer load power is 30W, a 15V adapter might
be current limited at 2.5A. If adapter voltage is less than
(30W/2.5A = 12V) when full power is drawn, the adapter
voltage will be pulled down by the constant 30W load
until it reaches a lower stable state where the switching
regulators can no longer supply full load. This situation
can be prevented by utilizing the DCDIV resistor divider,
set higher than the minimum adapter voltage where full
power can be achieved.
Input and Output Capacitors
We recommend the use of high capacity low ESR/ESL X5R
type ceramic capacitors. Alternative capacitors include
OSCON or POSCAP type capacitors. Aluminum electrolytic
capacitors are not recommended for poor ESR and ESL
reasons. Solid tantalum low ESR capacitors are acceptable,
but caution must be used when tantalum capacitors are
used for input or output bypass. High input surge currents
can be created when the power adapter is hot-plugged
into the charger or when a battery is connected to the
charger. Use only “surge robust” low ESR tantalums. Re-
gardless of which type of capacitor you use, after voltage
selection, the most important thing to meet is the ripple
current requirements followed by the capacitance value.
By the time you solve the ripple current requirements,
the minimum capacitance value is often met by default.
