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LT1306

Synchronous, Fixed Frequency Step-Up DC/DC Converter

Linear Technology 

LT1306

APPLICATIONS INFORMATION

Output Voltage Setting

The output voltage of the LT1306 is set with a resistive

divider, R1 and R2 (Figure 1 and Figure 5), from the output

to ground. The divider tap is tied to the FB pin. Current

through R2 should be significantly higher than the FB pin

input bias current (≤ 25nA). With R2 = 249k, the input bias

current of the error amplifier is 0.5% of the current in R1.

FB PIN

VO

( ) R1

VO = 1.24V

1+

R1

R2

( ) R2

R1 = R2

VO

1.24

–1

1306 F05

Figure 5. Feedback Resistive Divider

Synchronization and Shutdown

The S/S pin (Pin 8) can be used to synchronize the

oscillator or disconnect the load from the input. The S/S

pin is tied to the input (VIN > 1.8V) for normal operation.

The oscillator in the LT1306 can be externally synchro-

nized by driving the S/S pin with a pulse train (See the

graph “Maximum Allowable Rise Time of Synchronizing

Pulse” in the Typical Performance Characteristics). The

synchronization is positive edge triggered. The recom-

mended frequency of the external clock ranges from

425kHz to 500kHz. If synchronization results in switching

jitter, reducing the rising edge dv/dt of the external clock

pulse usually cures the problem.

Shutdown will be activated if the S/S pin voltage stays

below the shutdown threshold (0.45V) for more than

50µs. This shutdown delay is reset whenever the S/S pin

goes above the shutdown threshold.

Inductor

The value of the energy storage inductor L1 (Figure 1) is

usually selected so that the peak-to-peak ripple current is

less than 40% of the average inductor current. For 1- or

2-cell alkaline or single Li-Ion to 5V applications, 10µH to

20µH is recommended for the LT1306 running at 300kHz.

A 5µH to 10µH inductor can be used if the LT1306 is

externally synchronized at 500kHz.

The inductor should be able to handle the full load peak

inductor current without saturation. The peak inductor

current can be as high as 2A. This places a lower limit on

the core size of the inductor. Powder iron cores have

unacceptable core losses and are not suitable for high

efficiency applications. Most ferrite core materials have

manageable core losses and are recommended. Inductor

DC winding resistance (DCR) also needs to be considered

for efficiency. Usually there are trade-offs between core

loss, DCR, saturation current, cost and size.

For EMI sensitive applications, one may want to use

magnetically shielded or toroidal inductors to contain field

radiation. Table 1 lists a number of inductors suitable for

LT1306 applications.

Table 1. Inductors Suitable for Use with the LT1306

VENDOR

PART

NO.

VALUE MAX DCR CORE HEIGHT

(µH)

(Ω)

TYPE (mm)

BH Electronics 511-0033

5.0 0.023 Toroid 4.8

Coilcraft

DO3308-103 10

0.09

Open

3.0

DO3316-472 4.7 0.018 Open 5.2

DO3316-103 10

0.029

Open

5.2

DO3316-153 15

0.046

Open

5.2

Coiltronics CTX5-2

5

0.021 Toroid 6.0

CTX10-2

10

0.032 Toroid 6.0

Murata

LQN6C4R7 4.7 0.034 Open 5.0

Sumida

CDRH73-100 10

0.072 Magnetic 3.4

Shielding

CD43-4R7

4.7 0.109 Open 3.2

Capacitors

The output filter capacitor is usually chosen based on its

equivalent series resistance (ESR) and the acceptable

change in output voltage as a result of load transients. The

output voltage ripple at the switching frequency can be

estimated by considering the peak inductor current and

the capacitor ESR.

(IO)(VO)

IPEAK ≈ IIN ≈ VIN

( )( )( ) output ripple ≅ (ESR)(IPEAK) =

ESR IO

VIN

VO

9

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