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LTC3113

3113f

OPERATION

Internal Soft-Start

The LTC3113 buck-boost converter has an independent

internal soft-start circuit with a nominal duration of 2ms.

The converter remains in regulation during soft-start and

will therefore respond to output load transients which occur

during this time. In addition, the output voltage rise time

has minimal dependency on the size of the output capaci-

tor or load current during start-up. During soft-start, the

buck-boost is forced into PWM mode operation regardless

of the state of the BURST pin.

Thermal Shutdown

If the die temperature exceeds 155°C the LTC3113 buck-

boost converter will be disabled. All power devices are

turned off and the switch nodes will be forced into a high

impedance state. The soft-start circuit for the converter

is reset during thermal shutdown to provide a smooth

recovery once the overtemperature condition is eliminated.

When the die temperature drops to approximately 145°C

the LTC3113 will restart. For recommendations regarding

thermal design of the LTC3113 PCB, refer to the PCB Ther-

mal Considerations section in Applications Information.

Undervoltage Lockout

If the supply voltage decreases below 1.6V (typical) then

the LTC3113 buck-boost converter will be disabled and

all power devices are turned off. The soft-start circuit is

reset during undervoltage lockout to provide a smooth

restart once the input voltage rises above 1.7V (typical)

the undervoltage lockout increasing threshold.

When operating the LTC3113 at low input voltages, care

must be taken under heavy loads to prevent the part from

cycling into and out of UVLO. When operating at low input

voltages the voltage drop created by the source resistance

can trigger the UVLO, resetting the part. Operation near the

undervoltage lockout is not recommended, but if require-

ments dictate, the source resistance should be less than

100mV/IIN(MAX) (where IIN(MAX) is the maximum input

current) to ensure proper operation.

Inductor Damping

When the LTC3113 is in burst operation and sleep mode,

active circuits “damp” the inductor voltage through 165Ω

(typical) impedance from both SW1 and SW2 to ground

minimizing EMI.

PWM Mode Operation

When the BURST pin is held low, the LTC3113 buck-

boost converter operates in a ﬁxed-frequency pulse width

modulation (PWM) mode using voltage mode control. Full

output current is only available in PWM mode. A proprietary

switching algorithm allows the converter to transition

between buck, buck-boost, and boost modes without

discontinuity in inductor current. The switch topology for

the buck-boost converter is shown in Figure 1.

Figure 1. Buck-Boost Switch Topology

3113 F01

VIN

VOUT

When the input voltage is signiﬁcantly greater than the

output voltage, the buck-boost converter operates in

buck mode. Switch D turns on continuously and switch

C remains off. Switches A and B are pulse width modu-

lated to produce the required duty cycle to support the

output regulation voltage. As the input voltage decreases,

switch A remains on for a larger portion of the switching

cycle. When the duty cycle reaches approximately 85%,

the switch pair AC begins turning on for a small fraction

of the switching period. As the input voltage decreases

further, the AC switch pair remains on for longer durations

and the duration of the BD phase decreases proportionally.

As the input voltage drops below the output voltage, the

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