12
LT1307/LT1307B
1307fa
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 C
IN
must
be placed
close (<5mm) to the IC package. As little as 10mm of wire
or PC trace from C
IN
to V
IN
will cause problems such as
inability to regulate or oscillation. A 1µF 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 100µF aluminum electrolytic unit works well
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
I
2
R 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 10µH 0.57 Murata-Erie 2.0 Smallest Size
DO1608-103 10µH 0.16 Coilcraft 3.0
CD43-100 10µH 0.18 Sumida 3.2
CD54-100 10µH 0.10 Sumida 4.5 Best Efficiency
CTX32CT-100 10µH 0.50 Coiltronics 2.2 1210 Footprint
Capacitors
For single cell applications, a 10µF 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 10µF is needed.
For a typical 2-cell to 5V application, a 47µF 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 10µF ceramic in parallel with a low cost
(read high ESR) electrolytic capacitor, either tantalum or
aluminum, can be used instead. For through hole applica-
Figure 12. Recommended Component Placement. Traces
Carrying High Current Are Direct. Trace Area at FB Pin and V
C
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.
AA CELL
1
2
3
4
8
7
6
5
R1 R2
L
C
IN
D
LT1307
KEEP TRACES
OR LEADS SHORT!
V
OUT
C
OUT
C
C
GROUND
1307 F12
R
C
APPLICATIO S I FOR ATIO
WUUU
