6

LT1610

OPERATION

The LT1610 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 is closed around the output of the converter. When

70mV 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

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

30

µA. As output loading causes the FB voltage to de-

crease, A1’s output goes high, enabling the rest of the IC.

Switch current is limited to approximately 100mA 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 LT1610. Low fre-

quency 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 LT1610

is running continuously, peak switch current is controlled

on at the beginning of each switch cycle. When the sum-

mation of a signal representing switch current and a ramp

generator (introduced to avoid subharmonic oscillations at

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 either an external

ground or the internal RC network on the COMP pin (Pin

8). The typical values for the internal RC are 50k and 50pF.

LAYOUT

Although the LT1610 is a relatively low current device, its

high switching speed mandates careful attention to layout

for optimum performance. For boost converters, follow

the component placement indicated in Figure 3 for the best

results. C2’s negative terminal should be placed close to

Pin 4 of the LT1610. Doing this reduces switching currents

in the ground copper which keeps high frequency “spike”

noise to a minimum. Tie the local ground into the system

ground plane at one point only, using a few vias, to avoid

introducing dI/dt induced noise into the ground plane.

1

2

8

7

3

4

6

5

L1

C2

LT1610

GND

SHUTDOWN

R1

R2

MULTIPLE

VIAs

GROUND PLANE

1610 F03

+

C1

D1

+

Figure 3. Recommended Component Placement for Boost Converter. Note Direct High Current Paths Using

Ground Plane. Use Vias at One Location Only to Avoid Introducing Switching Currents into the Ground Plane