LTC3786 datasheet(21/44 Pages) LINER

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LT8710

8710f

For more information www.linear.com/LT8710

RFBX =

VOUT(FLOAT)–1.213V

83.7µA

RFLAG=RFBX •

1.213V

VOUT(BULK)– VOUT(FLOAT)

Battery Charging and C/10

A useful application for limiting the output current is to

charge a battery. When charging a battery such as a 12V

lead acid battery, it may be useful to charge to a bulk and

float voltage, in which case, the C/10 function of the FLAG

pin can be used. For decreasing charge currents, C/10

is detected when the IMON voltage falls below 666.5mV

(typical) and corresponds to an average ISP – ISN voltage

of 5mV (typical). For increasing charge currents, C/10 is

cleared when IMON gets above 727.5mV (typical) which

corresponds to an average ISP – ISN voltage of 10mV

(typical).

To set a bulk and float battery voltage, simply connect

a resistor from the FLAG pin to the FBX pin. When the

battery charging current is high (C/10 not detected), the

target output voltage is the bulk battery voltage as set by

the resistor connected between the FLAG and FBX pins.

OncethechargingcurrentdropssuchthatC/10isdetected,

the target output voltage drops to the float battery voltage

as set by the external FBX resistor. See Figure 10 below

on the FLAG pin connections and equations for setting the

bulk and float battery voltages. Note that in order to use

the C/10 feature, the MODE pin must be high to operate

in DCM at light loads.

applicaTions inForMaTion

Figure 10. FLAG Pin Connections and Equations

for Battery Charging

8710 F10

FBX

VOUT

FROM

CONTROLLER

VOUT

RFBX

LEAD ACID

BATTERY

1.213V

FLAG

100µs

ANTI-GLITCH

GND

COUT

83.7µA

RFLAG

IMON

DCM_EN

CHRG

666.5mV

Capacitor Charging

When the application is to charge a bank of capacitors

such as SuperCaps, the charging current is set by RSENSE2

and the FLAG pin isn’t necessarily needed as in the case

of charging a battery.

Temperature Dependent Output Voltage Using NTC

Resistor

Itmaybedesirabletoregulatetheconverter’soutputbased

on the ambient temperature. The INTVCC LDO regulated

voltage is 6.3V ± 1.6% (see Electrical Characteristics), and

a negative temperature coefficient (NTC) resistor can be

used to sum into the FBX pin to create an output voltage

that decreases with temperature. See Figure 11 for the

necessary connections.

The FBX voltages regulates to 1.213V (typical) for posi-

tive output voltages. For an accurate room temperature

output voltage, size the resistor divider off the INTVCC

pin to give 1.213V such that the current through R2 is

~0 at room temperature. Choose RNTC(25) ≤ 10kΩ and

use the equations below to calculate R1, RFBX, and VOUT

at room temperature and R2 for a desired VOUT change

over temperature.

VOUT(25)≅1.213V+83.7µA •RFBX+

FBX

•

1.213V – 6.3V •

R1+RNTC(25)









RNTC=RNTC(25)•e

β•

– 1

T25

(

)

∆VOUT = –6.3V •

RFBX

•R1•

R1+RNTC(T(MAX))

–

R1+RNTC(T(MIN))













R1+RNTC(T(MAX))

–

R1+RNTC(T(MIN))











1=R

NTC(25)

6.3–1.213V

1.213V

2 =

–6.3V

∆VOUT

•RFBX • •

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