LT1307CS8 datasheet(12/20 Pages) LINER

12 / 20 page

LT1307/LT1307B

APPLICATIONS INFORMATION

LAYOUT HINTS

The LT1307 switches current at high speed, mandating

careful attention to layout for proper performance.

You will

not get advertised performance with careless layouts.

Figure 12 shows recommended component placement.

Follow this closely in your PC layout. Note the direct path

of the switching loops. Input capacitor CIN must be placed

close (< 5mm) to the IC package. As little as 10mm of wire

or PC trace from CIN to VIN will cause problems such as

inability to regulate or oscillation. A 1

µF ceramic bypass

capacitor is the only input capacitance required

provided

the battery has a low inductance path to the circuit. The

battery itself provides the bulk capacitance the device

requires for proper operation. If the battery is located some

distance from the circuit, an additional input capacitor may

be required. A 100

µFaluminumelectrolyticunitworkswell

in these cases. This capacitor need not have low ESR.

COMPONENT SELECTION

Inductors

Inductors appropriate for use with the LT1307 must pos-

sess three attributes. First, they must have low core loss at

600kHz. Most ferrite core units have acceptable losses at

this switching frequency. Inexpensive iron powder cores

should be viewed suspiciously, as core losses can cause

significant efficiency penalties at 600kHz. Second, the

inductor must handle current of 500mA without saturat-

ing. This places a lower limit on the physical size of the unit.

Molded chokes or chip inductors usually do not have

enough core to support 500mA current and are unsuitable

for the application. Lastly, the inductor should have low

DCR (copper wire resistance) to prevent efficiency-killing

I2R losses. Linear Technology has identified several induc-

tors suitable for use with the LT1307. This is not an

exclusive list. There are many magnetics vendors whose

components are suitable for use. A few vendor’s compo-

nents are listed in Table 1.

Table 1. Inductors Suitable for Use with the LT1307

MAX

HEIGHT

PART

VALUE

DCR

MFR

(mm)

COMMENT

LQH3C100

µH

0.57

Murata-Erie

2.0

Smallest Size

DO1608-103

µH

0.16

Coilcraft

3.0

CD43-100

µH

0.18

Sumida

3.2

CD54-100

µH

0.10

Sumida

4.5

Best Efficiency

CTX32CT-100

µH

0.50

Coiltronics

2.2

1210 Footprint

Capacitors

For single cell applications, a 10

µF ceramic output capaci-

tor is generally all that is required. Ripple voltage in Burst

Mode can be reduced by increasing output capacitance.

For 2- and 3-cell applications, more than 10

µF is needed.

For a typical 2-cell to 5V application, a 47

µF to 100µF low

ESR tantalum capacitor works well. AVX TPS series (100%

surge tested) or Sprague (don’t be vague—ask for Sprague)

594D series are both good choices for low ESR capacitors.

Alternatively, a 10

µF ceramic in parallel with a low cost

(read high ESR) electrolytic capacitor, either tantalum or

aluminum, can be used instead. For through hole applica-

KEEP TRACES

OR LEADS SHORT!

CIN

COUT

VOUT

GROUND

1306 F12

LT1307

Figure 12. Recommended Component Placement. Traces

Carrying High Current Are Direct. Trace Area at FB Pin and VC

Pin is Kept Low. Lead Length to Battery Should Be Kept Short

OPERATION FROM A LABORATORY POWER SUPPLY

If a lab supply is used, the leads used to connect the circuit

to the supply can have significant inductance at the

LT1307’s switching frequency. As in the previous situa-

tion, an electrolytic capacitor may be required at the circuit

in order to reduce the AC impedance of the input suffi-

ciently. An alternative solution would be to attach the

circuit directly to the power supply at the supply terminals,

without the use of leads. The power supply’s output

capacitance will then provide the bulk capacitance the

LT1307 circuit requires.

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