8

LT1308A/LT1308B

1308abfa

TRACE A: LT1308A

AC COUPLED

800mA

50mA

TRACE B: LT1308B

AC COUPLED

200

μs/DIV

1308 F03

(CIRCUIT OF FIGURE 1)

Figure 3. LT1308A Exhibits Burst Mode Operation Output

Voltage Ripple at 50mA Load, LT1308B Does Not

OPERATION

The LT1308A combines a current mode, fixed frequency

PWM architecture with Burst Mode micropower operation

to maintain high efficiency at light loads. Operation can be

best understood by referring to the block diagram in Figure

2. Q1 and Q2 form a bandgap reference core whose loop

1V, the feedback voltage of 1.22V, along with an 80mV

drop across R5 and R6, forward biases Q1 and Q2’s base

collector junctions to 300mV. Because this is not enough

to saturate either transistor, FB can be at a higher voltage

comparator A1, A1’s output goes low, turning off all

circuitry except the input stage, error amplifier and low-

battery detector. Total current consumption in this state is

140

μA. As output loading causes the FB voltage to

decrease, A1’s output goes high, enabling the rest of the

IC. Switch current is limited to approximately 400mA

initially after A1’s output goes high. If the load is light, the

output voltage (and FB voltage) will increase until A1’s

output goes low, turning off the rest of the LT1308A. Low

frequency ripple voltage appears at the output. The ripple

frequency is dependent on load current and output capaci-

tance. This Burst Mode operation keeps the output regu-

lated and reduces average current into the IC, resulting in

high efficiency even at load currents of 1mA or less.

If the output load increases sufficiently, A1’s output

remains high, resulting in continuous operation. When the

LT1308A is running continuously, peak switch current is

is turned on at the beginning of each switch cycle. When

the summation of a signal representing switch current and

a ramp generator (introduced to avoid subharmonic oscil-

signal, comparator A2 changes state, resetting the flip-flop

and turning off the switch. Output voltage increases as

switch current is increased. The output, attenuated by a

resistor divider, appears at the FB pin, closing the overall

loop. Frequency compensation is provided by an external

ground.

Low-battery detector A4’s open-collector output (LBO)

pulls low when the LBI pin voltage drops below 200mV.

There is no hysteresis in A4, allowing it to be used as an

amplifier in some applications. The entire device is dis-

abled when the SHDN pin is brought low. To enable the

converter, SHDN must be at 1V or greater. It need not be

The LT1308B differs from the LT1308A in that there is no

hysteresis in comparator A1. Also, the bias point on A1 is

set lower than on the LT1308B so that switching can occur

at inductor current less than 100mA. Because A1 has no

hysteresis, there is no Burst Mode operation at light loads

and the device continues switching at constant frequency.

This results in the absence of low frequency output voltage

ripple at the expense of efficiency.

The difference between the two devices is clearly illus-

trated in Figure 3. The top two traces in Figure 3 shows an

LT1308A/LT1308B circuit, using the components indi-

cated in Figure 1, set to a 5V output. Input voltage is 3V.

Load current is stepped from 50mA to 800mA for both

circuits. Low frequency Burst Mode operation voltage

ripple is observed on Trace A, while none is observed on

Trace B.

At light loads, the LT1308B will begin to skip alternate

cycles. The load point at which this occurs can be de-

creased by increasing the inductor value. However, output

ripple will continue to be significantly less than the LT1308A

output ripple. Further, the LT1308B can be forced into

sinking 40

switching by causing A1’s output to go low.