LT1306 datasheet(8/16 Pages) LINER

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LT1306

OPERATIO

In boost operation, X5 drives the rectifier Q2 into satura-

tion. The voltage across the rectifier is VCESAT. As the

inductor current decreases, Q2’s base drive also de-

creases. X5 ceases supplying base current to Q2 when the

inductor current falls to zero.

If VIN > VO, Q2 will no longer be driven into saturation.

Instead the voltage across Q2 is allowed to increase so that

the inductor voltage reverses polarity as Q1 switches.

Since the inductor voltage is bipolar, volt-second balance

can be maintained regardless of the input voltage. The

LT1306 is therefore capable of operating as a step-down

regulator with the basic boost topology. Input

start-up current is also well controlled since the inductor

current cannot increase during Q1’s off-time with negative

inductor voltage.

The rectifier voltage drop depends on both the input and

the output voltages. Efficiency in the step-down mode is

less than that of a linear regulator. For sustained step-

down operation, the maximum output current will be

limited by the package thermal characteristics.

MODE

1306 F04

VO + 0.1V

VIN

BOOST

STEPDOWN

Figure 4. DC Transfer Characteristics of the Mode Control

Comparator Plotted with VIN as an Independent Variable.

VO is Considered Fixed.

MODE

1306 F03

VIN – 0.1V VIN

BOOST

STEPDOWN

Figure 3. DC Transfer Characteristics of the Mode Control

Comparator Plotted with VO as an Independent Variable.

VIN is Considered Fixed.

A hysteretic comparator in driver X5 controls the mode

of operation. DC transfer characteristics of the compara-

tor are shown in Figure 3 and Figure 4.

A logic low at the S/S pin (Pin 8) initiates shutdown. First,

all circuit blocks in the LT1306 are switched off. The

synchronous rectifier Q2 and its driver are kept on to

allow stored inductive energy to flow to the output. As VO

drops below VIN, the voltage across the rectifier Q2

increases so that the inductor voltage reverses. Inductor

current continues to fall to zero. Driver X5 then turns off

and the rectifier, Q2, becomes an open circuit. The

LT1306 dissipates only 9

µA in shutdown.

The LT1306 is guaranteed to start with a minimum VIN of

1.8V. Comparator A5 senses the input voltage and gen-

erates an undervoltage lockout (UVLO) signal if VIN falls

below this minimum. In UVLO, VC is pulled low and Q1

stops switching. The LT1306 draws 160

µA from the

input.

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