LTC4001 Datasheet, PDF (14/20 Pages) Linear Technology – 2A Synchronous Buck Li-Ion Charger

LTC4001
APPLICATIO S I FOR ATIO
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:
∆I_L
=
V_BAT
–
V_BAT2
V_IN
L•f
where V_BAT is the battery voltage, V_IN is the input voltage,
L is the inductance and f is the PWM oscillator frequency
(typically 1.5MHz). Maximum inductor ripple current oc-
curs at maximum V_IN and V_BAT = V_IN/2.
Peak inductor current will be:
I_PK = I_BAT + 0.5 • ∆I_L
where I_BAT is the maximum battery charging current.
When sizing the inductor make sure that the peak current
will not exceed the saturation current of the inductors.
Also, ∆I_L should never exceed 0.4(I_BAT) as this may
interfere with proper operation of the output short-circuit
protection comparator. 1.5µH provides reasonable induc-
tor ripple current in a typical application. With 1.5µH and
2A charge current:
2.85V – ^2.85V2
∆I_L
=
5.5V
1.5µH • 1.5MHz
= 0.61A_P-P
and
I_PK = 2.31A
Remote Sensing
For highest float voltage accuracy, tie GNDSENS and
BATSENS directly to the battery terminals. In a similar
fashion, tie BAT and PGND directly to the battery termi-
nals. This eliminates IR drops in the GNDSENS and
BATSENS lines by preventing charge current from flowing
in them.
Operation with a Current Limited Wall Adapter
Wall adapters with or without current limiting may be used
with the LTC4001, 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. For example, if the LTC4001 is pro-
grammed 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_BAT, is initially below V_TRIKL, the
trickle charge threshold (Figure 5). Battery charging be-
gins at approximately 50mA, well below the wall adapter
current limit so the voltage into the LTC4001 (V_IN) is the
wall adapter’s rated output voltage (V_ADAPTER). Battery
voltage rises eventually reaching V_TRIKL. The linear charger
shuts 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 PWM charger duty cycle ramps up to 100%
(the topside PMOS switch in the LTC4001 buck regulator
stays on continuously). As the battery voltage approaches
V_FLOAT, the float voltage error amplifier commands the
PWM charger to deliver less than I_LIMIT. The wall adapter
exits current limit and the V_IN jumps back up to V_ADAPTER.
4001f
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