6

LT1316

10

µs/DIV

1316 F02

Figure 2. Switching Waveforms

10

100

1000

10

100

1316 F03

Note: DC Current is the Peak Switch Current if the Power

Transistor had Zero Turn-Off Delay

AC COUPLED

200mV/DIV

5V/DIV

INDUCTOR

CURRENT

100mA/DIV

During the portion of the switch cycle when Q1 is turned

off, current is forced through D1 to C1 causing output

voltage to rise. This switching action continues until

output voltage rises enough to overcome A1’s hysteresis.

and is used to set up a constant current through R5. This

current also flows through R3 which sets the voltage at the

positive input of comparator A2. When Q1 turns on, the

Current through Q2 is equal to Q1’s current divided by 200.

When current through Q2 causes the voltage drop across

R4 and R3 to be equal, A2 changes state and resets the

oscillator, causing Q1 to turn off. Shutdown is accom-

plished by grounding the SHDN pin.

The low-battery detector A3 has its own 1.17V reference

and is always on. The open collector output device can sink

up to 500

µA. Approximately 35mV of hysteresis is built

into A3 to reduce “buzzing” as the battery voltage reaches

the trip level.

Current Limit

During active mode when the part is switching, current in

the inductor ramps up each switch cycle until reaching a

preprogrammed current limit. This current limit value

must be set by placing the appropriate resistor from the

using Figure 3 to locate the desired DC current limit and

Table 1 simplifies component selection for commonly

used input and output voltages. The methods used in

determining these values are discussed in more detail later

in this data sheet.

R2 + R1

R2

)

)

R1

R2

1316 EQF01

FB

LOAD

PEAK SWITCH

CURRENT

RESISTOR

INDUCTOR

CURRENT

2

5

10mA

36.8k

100

µH

80mA

2

5

25mA

18.2k

68

µH

165mA

2

5

50mA

10k

47

µH

320mA

2

5

75mA

6.81k

33

µH

500mA

5

12

100mA

6.81k

82

µH

490mA

5

28

1mA

75k

100

µH

56mA

5

28

5mA

22.1k

100

µH

140mA

5

28

10mA

10k

100

µH

270mA

Operation

To understand operation of the LT1316, first examine

Figure 1. Comparator A1 monitors FB voltage which is

When voltage at the FB pin drops below the reference

voltage (1.23V), A1’s output goes high and the oscillator

is enabled. The oscillator has an off-time fixed at 2

µs and

an on-time limited to 6.3

µs. Power transistor Q1 is cycled

on and off by the oscillator forcing current through the

inductor to alternately ramp up and down (see Figure 2).