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LT1304

Micropower DC/DC Converters with Low-Battery Detector Active in Shutdown

Linear Technology 

LT1304/LT1304-3.3/LT1304-5

U

OPERATIO

If the full power capability of the LT1304 is not required,

peak switch current can be limited by connecting a resistor

RLIM from the ILIM pin to ground. With RLIM = 22k, peak

switch current is reduced to approximately 500mA. Smaller

power components can then be used. The graph in Figure

6 shows switch current vs RLIM resistor value.

VOUT

2V/DIV

IIN

500mA/DIV

VSHDN

10V/DIV

1ms/DIV

1304 F05

Figure 5. Start-Up Response with 1µF/1MΩ Components

in Figure 2 Added. Input Current Is More Controlled. VOUT

Reaches 5V in 6ms. Output Drives 20mA Load

1000

900

800

700

600

bypass capacitor is required. If the input supply is close to

the IC, a 1µF ceramic capacitor can be used instead. The

LT1304 switches current in 1A pulses, so a low impedance

supply must be available. If the power source (for example,

a 2 AA cell battery) is within 1 or 2 inches of the IC, the

battery itself provides bulk capacitance and the 1µF ce-

ramic capacitor acts to smooth voltage spikes at switch

turn-on and turn-off. If the power source is far away from

the IC, inductance in the power source leads results in high

impedance at high frequency. A local high capacitance

bypass is then required to restore low impedance at the IC.

SHUTDOWN

VIN

VOUT

1

8

2

LT1304 7

3

6

4

5

+COUT

+CIN

500

400

10

100

RLIM (kΩ)

1000

1304 F06

Figure 6. Peak Switch Current vs RLIM Value

GND (BATTERY AND LOAD RETURN)

1304 F07

Figure 7. Suggested Layout for Best Performance. Input

Capacitor Placement as Shown Is Highly Recommended.

Switch Trace (Pin 4) Copper Area Is Minimized

Layout/Input Bypassing

The LT1304’s high speed switching mandates careful

attention to PC board layout. Suggested component place-

ment is shown in Figure 7. The input supply must have low

impedance at AC and the input capacitor should be placed

as indicated in the figure. The value of this capacitor

depends on how close the input supply is to the IC. In

situations where the input supply is more than a few

inches away from the IC, a 47µF to 100µF solid tantalum

Low-Battery Detector

The LT1304 contains an independent low-battery detector

that remains active when the device is shut down. This

detector, actually a hysteretic comparator, has an open

collector output that can sink up to 500µA. The compara-

tor also operates below the switcher’s undervoltage lock-

out threshold, operating until VIN reaches approximately

1.4V. Figure 8 illustrates the input/output characteristic of

the detector. Hysteresis is clearly evident in the figure.

8

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