Data Sheet

LT3652
12
3652fe
For more information www.linear.com/LT3652
ripple current in the charger, so it must have an adequate
ripple current rating. RMS ripple current (I
CVIN(RMS)
) is:
I
CVIN(RMS)
≅ I
CHG(MAX)
• (V
BAT
/ V
IN
)•([V
IN
/ V
BAT
] – 1)
1/2
,
where I
CHG(MAX)
is the maximum average charge current
(100mV/R
SENSE
). The above relation has a maximum at
V
IN
= 2 • V
BAT
, where:
I
CVIN(RMS)
= I
CHG(MAX)
/2
The simple worst-case of ½ • I
CHG(MAX)
is commonly
used for design.
Bulk capacitance is a function of desired input ripple volt
-
age (ΔV
IN
), and follows the relation:
C
IN(BULK)
= I
CHG(MAX)
• (V
BAT
/V
IN
)/∆V
IN
(µF)
Input ripple voltages above 0.1V are not recommended.
10µF is typically adequate for most charger applica-
tions.
Charge Current Programming
The LT3652 charger is configurable to charge at average
currents as high as 2A. Maximum charge current is set by
choosing an inductor sense resistor (R
SENSE
) such that
the desired maximum average current through that sense
resistor creates a 100mV drop, or:
R
SENSE
= 0.1/I
CHG(MAX)
where I
CHG(MAX)
is the maximum average charge current.
A 2A charger, for example, would use a 0.05Ω sense
resistor.
BOOST Supply
The BOOST bootstrapped supply rail drives the internal
switch and facilitates saturation of the switch transistor.
Operating range of the BOOST pin is 0V to 8.5V, as refer
-
APPLICATIONS INFORMATION
Figure 1. Programming Maximum Charge
Current Using R
SENSE
SW
BOOST
SENSE
BAT
R
SENSE
LT3652
3652 F01
enced to the SW pin. Connect a 1µF or greater capacitor
from the BOOST pin to the SW pin.
The voltage on the decoupling capacitor is refreshed
through a diode, with the anode connected to either the
battery output voltage or an external source, and the
cathode connected to the BOOST pin. Rate the diode
average current greater than 0.1A, and reverse voltage
greater than V
IN(MAX)
.
To refresh the decoupling capacitor with a rectifying diode
from the battery with battery float voltages higher than
8.4V, a >100mA Zener diode can be put in series with
the rectifying diode to prevent exceeding the BOOST pin
operating voltage range.