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LT3652

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APPLICATIONS INFORMATION

VIN / BOOST Start-Up Requirement

The LT3652 operates with a VIN range of 4.95V to 32V,

however, a start-up voltage requirement exists due to

the nature of the non-synchronous step-down switcher

topology used for the charger. If there is no BOOST supply

available, the internal switch requires (VIN – VSW) ≥ 3.3V

to reliably operate. This requirement does not exist if the

BOOST supply is available and (VBOOST – VSW) > 2V.

When an LT3652 charger is not switching, the SW pin is

at the same potential as the battery, which can be as high

as VBAT(FLT). As such, for reliable start-up, the VIN supply

must be at least 3.3V above VBAT(FLT). Once switching

begins and the BOOST supply capacitor gets charged

such that (VBOOST – VSW) > 2V, the VIN requirement no

longer applies.

In low VIN applications, the BOOST supply can be powered

by an external source for start-up, eliminating the VIN

start-up requirement.

VBAT Output Decoupling

An LT3652 charger output requires bypass capacitance

connected from the BAT pin to ground (CBAT). A 10μF

ceramic capacitor is required for all applications. In systems

where the battery can be disconnected from the charger

output, additional bypass capacitance may be desired for

visual indication for a no-battery condition (see the Status

Pins section).

If it is desired to operate a system load from the LT3652

charger output when the battery is disconnected, additional

bypass capacitance is required. In this type of application,

excessive ripple and/or low amplitude oscillations can oc-

cur without additional output bulk capacitance. For these

applications, place a 100μF low ESR non-ceramic capacitor

(chip tantalum or organic semiconductor capacitors such

as Sanyo OS-CONs or POSCAPs) from BAT to ground,

in parallel with the 10μF ceramic bypass capacitor. This

additional bypass capacitance may also be required in

systems where the battery is connected to the charger

with long wires. The voltage rating of CBAT must meet or

exceed the battery float voltage.

Inductor Selection

The primary criterion for inductor value selection in an

LT3652 charger is the ripple current created in that inductor.

Once the inductance value is determined, an inductor must

also have a saturation current equal to or exceeding the

maximum peak current in the inductor. An inductor value

(L), given the desired amount of ripple current (ΔIMAX)

can be approximated using the relation:

L = (10 RSENSE / ΔIMAX) • VBAT(FLT) •

[1 – (VBAT(FLT) / VIN(MAX))]

(μH)

In the above relation, ΔIMAX is the normalized ripple cur-

rent, VIN(MAX) is the maximum operational voltage, and

VF is the forward voltage of the rectifying Schottky diode.

Ripple current is typically set within a range of 25% to

35% of ICHG(MAX), so an inductor value can be determined

by setting 0.25 < ΔIMAX < 0.35.

Figure 2. Zener Diode Reduces Refresh

Voltage for BOOST Pin

BOOST

SENSE

BAT

LT3652

3652 F02

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