5

LT1301

TEST CIRCUITS

Oscillator Test Circuit

2V

100

µF

SEL

SENSE

GND

PGND

SHDN

SW

LT1301

100

Ω

5V

LT1301 TC

Operation of the LT1301 is best understood by referring to

the Block Diagram in Figure 2. When A1’s negative input,

related to the Sense pin voltage by the appropriate resis-

tor-divider ratio is higher that the 1.25V reference voltage,

A1’s output is low. A2, A3 and the oscillator are turned off,

drawing no current. Only the reference and A1 consume

current, typically 120

µA. When A1’s negative input drops

below 1.25V, overcoming A1’s 6mV hysteresis, A1’s out-

put goes high enabling the oscillator, current comparator

A2, and driver A3. Quiescent current increases to 2mA as

the device prepares for high current switching. Q1 then

turns on in controlled saturation for (nominally) 5.3

µs or

until comparator A2 trips, whichever comes first. After a

fixed off-time of (nominally) 1.2

µs, Q1 turns on again. The

LT1301’s switching causes current to alternately build up

in L1 and dump into output capacitor C2 via D1, increasing

the output voltage. When the output is high enough to

cause A1’s output to go to low, switching action ceases.

enough to force A1’s output high, and the entire cycle

repeats. Figure 4 details relevant waveforms. A1’s cycling

causes low-to-mid-frequency ripple voltage on the output.

Ripple can be reduced by making the output capacitor

large. The 33

µF unit specified results in ripple of 100mV to

200mV on the 12V output. A 100

µF capacitor will decrease

ripple to 50mV. If operating at 5V ouput a 0.1

µF ceramic

capacitor is required at the Sense pin in addition to the

electrolytic.

If switch current reaches 1A, causing A2 to trip, switch on-

time is reduced and off-time increases slightly. This allows

continuous mode operation during bursts. A2 monitors

the voltage across 3

Ω resistor R1 which is directly related

to the switch current. Q2’s collector current is set by the

emitter-area ratio to 0.6% of Q1’s collector current. When

R1’s voltage drop exceeds 18mV, corresponding to 1A

switch current, A2’s output goes high, truncating the on-

time portion of the oscillator cycle and increasing off-time

to about 2

µs as shown in Figure 3, trace A. This pro-

to ground, causing 15

µA to flow through R2 into Q3’s

collector. Q3’s current causes a 10.4mV drop in R2 so that

only an additional 7.6mV is required across R1 to turn off

the switch. This corresponds to a 400mA switch current

as shown in Figure 3, trace B. The reduced peak switch

can be increased by doing this provided that the accom-

panying reduction in full load current is acceptable. Lower

peak currents also extend alkaline battery life due to the

alkaline cell’s high internal impedance.

TRACE A

500mA/DIV

OPEN

TRACE B

500mA/DIV

GROUNDED

20

µs/DIV