10

LT1302/LT1302-5

2ms/DIV

1302 F07c

100mV/DIV

AC COUPLED

Figure 7c. 0.01

Underdamped Response Requires Series R

510mA

10mA

LT1302 to operate in current mode even at light load. To

to gound. More conservative frequency compensation

must be used when in this mode. A 0.1

µF capacitor and

adequate. Low frequency Burst Mode ripple can be

reduced or eliminated using this technique in many appli-

cations.

To illustrate, the transient load response of Figure 8’s

circuit is pictured without and with R1. Figure 8a shows

output voltage and inductor current without the resistor.

Note the 6kHz burst rate when the converter is delivering

25mA. By adding the 3.3k resistor, the low frequency

bursting is eliminated, as shown in Figure 8b. This feature

is useful in systems that contain audio circuitry. At very

light or zero load, switching frequency drops and eventu-

1ms/DIV

1302 F08a

100mV/DIV

AC COUPLED

Causes 220mV Undershoot

525mA

25mA

INDUCTOR

CURRENT

1A/DIV

100mV/DIV

AC COUPLED

2ms/DIV

1302 F07f

510mA

10mA

Figure 7f. 0.01

µF, 24k Series RC

Results in Optimum Response

2ms/DIV

1302 F07e

100mV/DIV

AC COUPLED

510mA

10mA

Figure 7e. 0.01

µF, 10k Series RC Shows Good

Transient Response. Slight Underdamping

Still Noticeable

2ms/DIV

1302 F07d

100mV/DIV

AC COUPLED

Figure 7d. 0.1

µF with 10k Series RC.

Classic Overdamped Response

510mA

10mA

SENSE

SW

10

µH

5V

600mA

MBRS130LT3

PGND

0.1

µF

LT1302-5

GND

4.7k

R1

3.3k

1302 F08

2.5V

+

220

µF

10V

+

220

µF

10V

+

C1

330

µF

0.1

µF

Figure 8. Addition of R1 Eliminates Low Frequency

Output Ripple in This 2.5V to 5V Boost Converter