lt3651-4.2 Linear Technology Corporation, lt3651-4.2 Datasheet - Page 11

• Manufacturer Part Number: lt3651-4.2
• Description: Monolithic 4a High Voltage Li-ion Battery Charger Features
• Manufacturer: Linear Technology Corporation

applicaTions inForMaTion
OSC Frequency
A precision resistor to ground sets the LT3651 switcher
oscillator frequency, f
of the frequency value. Typically this frequency is in the
200kHz to 1MHz range. Power consideration may neces-
sitate lower frequency operation especially if the charger
is operated with very high voltages. Adjustability also
allows the user to position switching harmonics if their
system requires.
The timing resistor, R
Set R
V
The LT3651 is biased directly from the charger input supply
through the V
currents, so a high quality, low ESR decoupling capacitor
is required to minimize voltage glitches on V
decoupling capacitor (C
ripple current in the charger. Size is determined by input
ripple voltage with the following equation:
where ∆V
charge current and f is the oscillator frequency. A good
starting point for ∆V
with V
I
MAX
IN
R
C
C
Input Supply
T
IN BULK
IN BULK
= 4A and a 1MHz oscillator frequency:
T
(
BAT
(
to 54.9k for 1MHz operation.
=
f
OSC
high and V
IN
)
)
54 9 .
is the input ripple, I
=
=
(
IN
MHz
1 0 1 8
pin. This supply provides large switched
f
OSC
4 4 2
• . •
• .
) ( )
(
Ω
IN
IN
MHz
I
T
MAX
k
is 0.1V. Worst-case conditions are
OSC
at minimum. So for a 8V V
, value is set by the following:
VIN
=
)
, permitting user adjustability
•
t
•
) absorbs all input switching
21
V
∆
BAT
µF
V
IN
•
MAX
V
IN
is the maximum
( )
µF
IN
. The V
IN(MIN)
IN
,
The capacitor must have an adequate ripple current rating.
RMS ripple current, I
which has a maximum at V
= I
capacitor RMS rating of 2A.
Boost Supply
The BOOST bootstrapped supply rail drives the internal
switch and facilitates saturation of the high side switch
transistor. The BOOST voltage is normally created by con-
necting a 1µF capacitor from the BOOST pin to the SW
pin. Operating range of the BOOST pin is 2V to 4.5V, as
referenced to the SW pin.
The boost capacitor is normally charged via a diode con-
nected from the battery or an external source through the
low side switch. Rate the diode average current greater than
0.1A and its reverse voltages greater than V
If an external supply that is greater than the input is avail-
able (V
bootstrap capacitor and diode.
V
The LT3651 operates with a V
charger begins a charging cycle when the detected battery
voltage is below the 4.1V auto-restart float voltage and
the part is enabled.
When V
charged, the high side switch would normally not have
sufficient head room to start switching. During normal
operation the low side switch is deactivated when charge
current is very low to prevent reverse current in the inductor.
IN
I
CHARGE(MAX)
CVIN RMS
,V
BOOST
BOOST
(
IN
is below 6.3V and the BOOST capacitor is un-
)
Start-Up Requirement
– V
≈
/2. In the example above that requires a
I
CHARGE MAX
IN
> 2V), it may be used in place of the
CVIN(RMS)
(
IN
)
= 2 • V
IN
•
is approximated by:

 
range of 4.8V to 32V. The
V
V
BAT
IN
LT3651-4.2
BAT

 
, where I
•
V
IN(MAX)
V
B
IN
A A T
CVIN(RMS)
– 1

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