LTC4001-1 Linear Technology, LTC4001-1 Datasheet - Page 14

• Manufacturer Part Number: LTC4001-1
• Description: 2A Synchronous Buck Li-Ion Charger
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC4001-1
Inductor Selection
A high (1.5MHz) operating frequency was chosen for the
buck switcher in order to minimize the size of the inductor.
However, take care to use inductors with low core losses
at this frequency. A good choice is the IHLP-2525AH-01
from Vishay Dale.
To calculate the inductor ripple current:
where V
L is the inductance and f is the PWM oscillator frequency
(typically 1.5MHz). Maximum inductor ripple current oc-
curs at maximum V
Peak inductor current will be:
where I
When sizing the inductor make sure that the peak current
will not exceed the saturation current of the inductors.
Also, ΔI
terfere with proper operation of the output short-circuit
protection comparator. 1.5μH provides reasonable inductor
ripple current in a typical application. With 1.5μH and 2A
charge current:
and
14
I
I
PK
PK
I
I
L
L
= 2.31A
= I
=
=
BAT
BAT
L
BAT
2.85V –
V
1.5μH • 1.5MHz
should never exceed 0.4(I
BAT
is the maximum battery charging current.
is the battery voltage, V
+ 0.5 • ΔI
L • f
–
V
BAT
2.85V
V
5.5V
IN
IN
2
and V
L
2
= 0.61A
BAT
= V
P-P
IN
IN
BAT
/2.
is the input voltage,
) as this may in-
Remote Sensing
For highest fl oat voltage accuracy, tie GNDSENS and
BATSENS directly to the battery terminals. In a similar fash-
ion, tie BAT and PGND directly to the battery terminals. This
eliminates IR drops in the GNDSENS and BATSENS lines
by preventing charge current from fl owing in them.
Operation with a Current Limited Wall Adapter
Wall adapters with or without current limiting may be used
with the LTC4001-1, however, lowest power dissipation
battery charging occurs with a current limited wall adapter.
To use this feature, the wall adapter must limit at a current
smaller than the high rate charge current programmed
into the LTC4001-1. For example, if the LTC4001-1 is
programmed to charge at 2A, the wall adapter current
limit must be less than 2A.
To understand operation with a current limited wall adapter,
assume battery voltage, V
trickle charge threshold (Figure 5). Battery charging begins
at approximately 50mA, well below the wall adapter current
limit so the voltage into the LTC4001-1 (V
adapter’s rated output voltage (V
rises eventually reaching V
off, the PWM (high rate) charger turns on and a soft-
start cycle begins. Battery charging current rises during
the soft-start cycle causing a corresponding increase in
wall adapter load current. When the wall adapter reaches
current limit, the wall adapter output voltage collapses
and the LTC4001-1 PWM charger duty cycle ramps up to
100% (the topside PMOS switch in the LTC4001-1 buck
regulator stays on continuously). As the battery voltage
approaches V
mands the PWM charger to deliver less than I
wall adapter exits current limit and the V
FLOAT
, the fl oat voltage error amplifi er com-
BAT
TRIKL
, is initially below V
. The linear charger shuts
ADAPTER
). Battery voltage
IN
jumps back up
IN
) is the wall
LIMIT
TRIKL
. The
40011fa
, the