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LTC4020

4020fd

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DC/DC CONVERTER SECTION

Output Voltage Programming

The LTC4020 DC/DC converter maximum output volt-

age, or voltage safety limit, is set by an external feedback

resistive divider, providing feedback to the VFBMAX pin.

This divider sets the output voltage that the converter

will servo to when the PowerPath FET is high impedance,

which occurs after charge cycle termination or during a

charge cycle fault.

applicaTions inForMaTion

PGND. Both nodes on the sense resistor must be Kelvin

connected to the IC, via the pins SENSBOT and SENSGND.

Both of these sense resistors must be of equal value, and

that value programs the switched inductor maximum

average current in the DC/DC converter inductor (ILMAX)

such that:

RSENSEA =RSENSEB =

0.05

ILMAX

When the converter is stepping down, or operating in buck

mode, the inductor current will be roughly equivalent to

theconverteroutputcurrent.Inputsupplycurrent(IIN)will

be less than the inductor current (IL), such that:

IL ~IIN •

VIN

VOUT









When the converter is stepping up, or operating in boost

mode, the inductor current will be roughly equivalent to

the converter input current. Inductor current (IL) will be

greater than output current (IOUT), such that:

IL ~IOUT •

VOUT

VIN









Overcurrent Detection

The LTC4020 also contains an overcurrent detection

circuit that monitors the low side current sense resistor,

or SENSBOT–SENSGND input. Should that circuit detect

a voltage on that input that is less than –150mV or greater

than 100mV, or roughly 3x the maximum average current,

all of the switches are disabled for four (4) clock cycles.

Parasitic inductances on non-ideal layouts and or body-

diode commutation charge can cause voltage spikes

across the sense resistor at the beginning of synchronous

FET conduction. The LTC4020 overcurrent circuitry is

somewhat resistant to these leading edge spikes but, in

some cases, the overcurrent circuit can be prematurely

triggered. This is identified by the repeated 4-cycle

switch off-time that occurs should premature triggering

occur. Placing a ceramic capacitor across the SENSBOT–

SENSGND input pins near the IC will generally eliminate

Figure 4. VOUT Safety Limit Programming

Theresultantfeedbacksignaliscomparedwiththeinternal

2.75V voltage reference by the converter error amplifier.

The output voltage is given by the equation:

VOUT =2.75V • 1+

RMAX1

RMAX2









where RMAX1 and RMAX2 are defined as in Figure 4.

The values for RMAX1 and RMAX2 are typically the same as

those used for the divider that programs battery voltage

(to the VFB pin; see Battery Charger section), to yield a

DC/DC converter maximum regulation voltage, or safety

limit, that is 10% higher than the battery charge voltage.

RSENSEA, RSENSEB: DC/DC Converter Current

Programming

The LTC4020 performs inductor current sensing via two

resistors connected in series with the VIN side switches

(see Figure 1). The high side sense resistor (RSENSEA) is

connected between VINandthedrainofthetopsideswitch

FET (A). Both nodes on the sense resistor must be Kelvin

connected to the IC via the pins SENSVIN and SENSTOP.

Likewise,thelowsidesenseresistor(RSENSEB)isconnected

between the source of the bottom side switch FET (B) and

4020 F04

VOUT

RMAX1

RMAX2

VFBMAX

LTC4020

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