LTC4121 datasheet(13/30 Pages) LINER

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LTC4121/LTC4121-4.2

4121fc

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OPERATION

When no power source is applied to VIN,forexamplewhen

using a solar panel source and the panel is in the dark, the

MPPTpindividerdrainspowerfromthebatterythroughthe

body diode of the top side switch of the switching regula-

tor. To eliminate this leakage path, the MPPT divider may

be connected to the anode of the Schottky diode that is in

series with the panel, for examples see Figures 1, 9, or 10.

For example, consider charging a battery from a source

withanopen-circuitvoltageof30Vandasourceimpedance

of 120Ω. This resistive supply has a short circuit current

of 250mA, and the peak available power of 1.875W oc-

curs with a load of 125mA at 50% of VOC. To program the

LTC4120 to optimize the available power for this source

simply program VMP/VOC to 50% by selecting the MPPT

resistivedividergainKR=0.2.Thisisobtainedwitharesis-

tive divider as shown in Figure 4 with RMPPT2 = RMPPT1/4.

With standard 1% resistors this is approximated with

RMPPT1 = 402k, and RMPPT2 = 100k.

If the MPPT pin sees excess capacitance to GND, this may

affect switching regulator stability. In such cases, one may

optionally add a 50pF to 150pF lead capacitor (CMPPT) as

shown in Figure 4.

The sampling of VOC is done at an extremely low duty

cycle so as to have minimum impact on the average

charge current. The time between sample events, TMP,

is typically about 30 seconds, with an idle time, PWMP,

of about 36ms to allow the source to recover to its

open-circuit voltage through the time constant associ-

ated with the input decoupling capacitor CIN. The time

constant for the source to recover to its open-circuit

voltage must be kept below the idle period. Limit the

input capacitor to 10µF to avoid increasing the source

recovery time.

Programming the Battery Float Voltage

For the LTC4121, the battery float voltage is programmed

by placing a resistive divider from the battery to FB and

FBG as shown in Figure 5. The battery float voltage is

programmable anywhere from 3.5V up to 18V. The pro-

grammable battery float voltage, VFLOAT, is then governed

by the following equation:

VFLOAT = VFB(REG) •

RFB1+RFB2

(

)

RFB2

where VFB(REG) is typically 2.4V.

Due to the input bias current (IFB) of the voltage error amp

(V-EA), care must also be taken to select the Thevenin

equivalent resistance of RFB1//RFB2 close to 588kΩ. Start

by calculating RFB1 to satisfy the following relations:

RFB1 =

VFLOAT • 588k

VFB(REG)

Find the closest 0.1% or 1% resistor to the calculated

value. With RFB1 calculate:

RFB2 =

VFB(REG) •RFB1

VFLOAT − VFB(REG)

−1000Ω

Where 1000Ω represent the typical value of RFBG. This is

the resistance of the FBG pin which serves as the ground

return for the battery float voltage divider.

Once RFB1 and RFB2 are selected re-calculate the value of

VFLOAT obtained with the resistors available. If the error

Figure 4. MPPT Resistive Divider

Figure 5. Programming the Float Voltage with LTC4121

4121 F05

BAT

22µF

RFB1

VFLOAT

RFB2

Li-Ion

FBG

ENABLE

IFB

LTC4121

4121 F04

RMPPT1

CMPPT

(OPTIONAL)

RMPPT2

IMPPT

MPPT

GND

LTC4121

LTC4121 Datasheet(PDF) 13 Page - Linear Technology

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