LT1776CS8 datasheet(18/20 Pages) LINER

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LT1776

Figure 7b shows that efficiency is typically maintained at

75% or better down to a load current of 10mA. Even at a

load of 1mA, efficiency is still a respectable 58% to 68%,

depending on VIN.

Resistor divider R1/R2 is still present, but does not

directly influence output voltage. It is chosen to ensure

that the LT1776 delivers high output current throughout

the voltage regulation range. Its presence is also required

to maintain proper short-circuit protection. Transistors

Q1, Q2 and resistor R7 form a high VIN, low quiescent

current voltage regulator to power U2.

Wide VIN Range, High Efficiency Battery Charger

The circuit on the final page of this data sheet shows the

LT1776 configured as a constant-current/constant-volt-

age battery charger. An LT1620 rail-to-rail, current sense

amplifier (U2) monitors the differential voltage across

current sense resistor R4. As this equals and exceeds the

voltage set across resistor R5 in the R5/R6 divider, the

LT1620 responds by sinking current at its IOUT pin. This is

connected to the VC control node of the LT1776 and

therefore acts to reduce the amount of power delivered to

the load. The overall constant-current/constant-voltage

behavior can be seen in the graph titled Battery Charger

Output Voltage vs Output Current.

Target voltage and current limits are independently pro-

grammable. Output voltage, presently 6V, is set by the

R1/R2 divider and the internal reference of the LT1776.

Output current, presently 200mA, is set by current sense

resistor R4 and the R5-R6 divider.

The circuit, as shown, accommodates an input voltage

range of 10V to 30V. The accompanying graphs display

efficiency for input voltages of 12V and 24V. The upper

input voltage limit of 30V is determined not by the LT1776,

but by the LT1121-5 regulator (U3). (A regulated 5V is

required by the LT1620.) This regulator was chosen for its

micropower behavior, which helps maintain good overall

efficiency. However, the basic catalog part is only rated to

30V. Substitution of the industry standard LM317, for

example, extends the allowable input voltage to 40V (or

more with the HV part), but its greater quiescent current

drain degrades efficiency from that shown.

A related concern in charger applications is the current

drain seen at the battery when charger power is removed.

Strictly speaking, this can occur in three separate ways:

the VIN supply can go to zero (VIN = short circuit), the VIN

supply can be disconnected (VIN = open circuit) or the

SHDN function can be asserted. The worst-case is gener-

ally VIN = 0V, and this situation will be assumed.

A diode is then required in the battery charger power path

to prevent reverse current flow. There are three logical

places for this diode. The first is directly in series with the

VSW node. This has the advantage of smallest efficiency

penalty, as the diode forward drop subtracts from the

input voltage. A disadvantage is that the battery must still

power the LT1776 VCC pin, yielding a current drain of

several mA. In this position the diode is called upon to

switch on and off rapidly, so a Schottky type, similar to that

used as the freewheeling diode (D1), is recommended.

Placing the diode between output filter capacitor C2 and

feedback divider R1/R2 limits the current drain to only the

current drawn by the feedback divider, perhaps 100

µA or

so. However, the efficiency penalty is greater, as the diode

forward drop is now in series with the output voltage.

When absolute minimal battery drain is required, the diode

may be placed between the R1/R2 feedback divider and

the battery itself. This limits current drain to just the

reverse leakage of the diode. In this case the feedback

divider must be adjusted for the nominal forward drop of

the diode. In either of these positions, a Schottky diode will

offer the least efficiency penalty, but a standard silicon

diode can be used in the most cost sensitive applications.

TYPICAL APPLICATIONS

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