LTC3541EDD-3 查看數據表（PDF） - Linear Technology

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LTC3541EDD-3

High Efficiency Buck + VLDO Regulator

Linear Technology 

LTC3541-3

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OPERATIO

When the MODE pin is driven to a logic high the LTC3541-3

operates in Pulse-Skip mode for low output voltage ripple.

In this mode, the LTC3541-3 continues to switch at a

constant frequency down to very low currents, where it

will begin skipping pulses used to control the main (top)

switch to maintain the proper average inductor current.

If the input supply voltage is decreased to a value ap-

proaching the output voltage, the duty cycle of the buck

is increased toward maximum on-time and 100% duty

cycle. The output voltage will then be determined by the

input voltage minus the voltage drop across the main

switch and the inductor.

VLDO/Linear Regulator Loop

In the LTC3541-3, the VLDO and linear regulator loops

consist of an amplifier and N-channel MOSFET output

stages that servo the output to maintain a regulator output

voltage, LVOUT. Loop configurations enabling the VLDO or

the linear regulator are stable with an output capacitance

as low as 2.2µF and as high as 100µF. Both the VLDO

regulator and the linear regulators are capable of operating

with an input voltage, VIN, as low as 3V.

The VLDO regulator is designed to provide up to 300mA

of output current at a very low LVIN to LVOUT voltage. This

allows a clean, secondary, analog supply voltage to be

provided with a minimum drop in efficiency. The VLDO

regulator is provided with thermal protection that is de-

signed to disable the VLDO function when the output, pass

transistor’s junction temperature reaches approximately

160°C. In addition to thermal protection, short-circuit

detection is provided to disable the VLDO function when

a short-circuit condition is sensed. This circuit is designed

such that an output current of approximately 1A can be

provided before this circuit will trigger. As detailed in the

Electrical Characteristics, the VLDO regulator will be out

of regulation when this event occurs. Both the thermal

and short-circuit faults, when detected, are treated as

catastrophic fault conditions. The LTC3541-3 will be reset

upon the detection of either event.

The N-channel MOSFET incorporated in the VLDO regulator

has its drain connected to the LVIN pin as shown in Figure

1. To ensure reliable operation, the LVIN voltage must be

stable before the VLDO regulator is enabled. For the case

where the voltage on LVIN is supplied by the buck regula-

tor, the internal power supply sequencing logic assures

voltages are applied in the appropriate manner. For the

case where the buck is enabled before the VLDO regula-

tor and an external supply is used to power the LVIN pin,

the voltage on LVIN pin must be stable 1ms before the

ENVLDO pin is brought from a low to a high. Further, the

externally supplied LVIN must be reduced in conjunction

with VIN whenever VIN is pulled low or removed.

The linear regulator is designed to provide a lower output

current (30mA) than that available from the VLDO regula-

tor. The linear regulator’s output, pass transistor has its

drain tied to the VIN rail. This allows the linear regulator

to be turned on prior to, and independent of, the buck

regulator which ordinarily drives the VLDO regulator. The

linear regulator is provided with thermal protection that

is designed to disable the linear regulator function when

the output pass transistor’s junction temperature reaches

approximately 160°C. In addition to thermal protection,

short-circuit detection is provided to disable the linear

regulator function when a short-circuit condition is sensed.

This circuit is designed such that an output current of

approximately 120mA can be provided before this circuit

will trigger. As detailed in the Electrical Characteristics,

the linear regulator will be out of regulation when this

event occurs. Both the thermal and short-circuit faults are

treated as catastrophic fault conditions. The LTC3541-3

will be reset upon the detection of either event.

The N-channel MOSFET incorporated in the linear regulator

has its drain connected to the VIN pin as shown in Figure

1. The size of these MOSFETs and their associated power

bussing is designed to accommodate 30mA of DC current.

Currents above this value can be supported for short periods

as stipulated in the Absolute Maximum Ratings.

35413fc

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