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dc1977afb

DEMO MANUAL

DC1977A-A/DC1977A-B

theory of operation

The LTC4121EUD-4.2/LTC4121EUD is a 4.4V ~ 40V input

buck topology battery charger with maximum power point

tracking (MPPT) for use with PV cells or highly resistive

power supplies. The buck-topology charger uses current

mode control for stable operation.

LTC4121EUD ENABLE

The LTC4121 can be enabled or disabled via the RUN pin,

and this functionality can be accessed via JP2, the RUN

jumper. When JP2 is in the “ENABLE” position, R3 and

R4 ensure that the LTC4121 is not enabled until Vin is

greater than 4.4V.

Note: Do not float the LTC4121 RUN pin. Operate

the demo board with JP2 in either the DISABLED or

ENABLED position.

Buck Charger

The heart of the LTC4121EUD is the buck-topology bat-

tery charger. The buck-topology charger is a synchronous,

current-mode-control regulator with N-channel FETs. The

use of N-channel FETs minimizes conduction losses, and

requires only a single external 0.022µF capacitor to gener-

ate the high-side gate drive.

The LTC4121EUD can charge up to four Li-Ion cells in

series, and supports a maximum battery voltage of 18V.

The LTC4121EUD-4.2 is optimized for charging a single

Li-Ion cell to a fixed cell voltage of 4.2V.

The current in the buck inductor passes through a small

on-die resistor for current measurement, and then goes

back out to the BAT pin. The battery is connected to the

BAT pin; this allows the LTC4121EUD to measure not only

the cycle-by-cycle current, but also the average current.

The cycle-by-cycle current is used by the current-mode

buck regulator, and the average current is the battery

MPPT function does not reduce the current.

The buck regulator acts as a current source when the

battery is in the constant-current charging region and as

a classic voltage output buck regulator when the battery

is in the constant-voltage charging region.

The battery charge current is programmed by

= 986 •

1.227V

0.4

= 3.01kΩ

The LTC4121EUD provides a switching frequency select

pin, FREQ, to select between 750kHz and 1.5MHz; this

function is accessed by JP4, the “FREQ” jumper.

Note: Do not float the LTC4121 FREQ pin. Operate the

demo board with JP3 in either the 750kHz or 1.5MHz

position.

and the switching frequency at 750kHz (T = 1.333µs), The

duty cycle is 86% for V(BAT) = 3.6V, not the 72% duty

expected from a buck regulator. When the battery voltage

is 3.6V, the charger is in constant-current mode, so the

control loop is forcing the output of the buck regulator

to the voltage necessary to push 400mA into the battery.

This “effective” voltage, 5 • 0.86 = 4.3V, is the voltage

necessary to ensure that a 400mA average current is flow-

ing through the on-die sense resistor.

Figure 3 shows the same nodes as Figure 2, but with

duty cycle is ≈ 200ns/1.333µs, or 15%, but the period is

2.7µs. This is because the LTC4121 minimum on time was

greater than that needed to achieve 4.3V, and the LTC4121

starts to pulse skip to get the necessary average duty

cycle. The average duty cycle is 300ns/2.667µs = 11%.

This produces an output voltage of 4.3V, so the battery

still charges at 400mA.

Maximum Power Point Tracking (MPPT)

The LTC4121EUD provides a maximum-power-point

tracking (MPPT) function for use with PV cells or highly