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LTC3528-2

1A, 2MHz Synchronous Step-Up DC/DC Converter in 2mm × 3mm DFN

Linear Technology 

LTC3528-2

OPERATION (Refer to Block Diagram)

Error Amplifier

The error amplifier is a transconductance type. The nonin-

verting input is internally connected to the 1.20V reference

and the inverting input is connected to FB. Clamps limit

the minimum and maximum error amp output voltage for

improved large-signal transient response. Power converter

control loop compensation is provided internally. A voltage

divider from VOUT to ground programs the output voltage

via FB from 1.6V to 5.25V.

VOUT

=

1.20V

•

1+

R2

R1

Current Sensing

Lossless current sensing converts the peak current signal

of the N-channel MOSFET switch into a voltage which

is summed with the internal slope compensation. The

summed signal is compared to the error amplifier output

to provide a peak current control command for the PWM.

Current Limit

The current limit comparator shuts off the N-channel MOS-

FET switch once its threshold is reached. The current limit

comparator delay to output is typically 60ns. Peak switch

current is limited to approximately 1.5A, independent of

input or output voltage, unless VOUT falls below 0.7V, in

which case the current limit is cut in half.

Zero Current Comparator

The zero current comparator monitors the inductor cur-

rent to the output and shuts off the synchronous rectifier

when this current reduces to approximately 20mA. This

prevents the inductor current from reversing in polarity,

improving efficiency at light loads.

Synchronous Rectifier

To control inrush current and to prevent the inductor

current from running away when VOUT is close to VIN, the

P-channel MOSFET synchronous rectifier is only enabled

when VOUT > (VIN + 0.24V).

Anti-Ringing Control

The anti-ringing control connects a resistor across the

inductor to prevent high frequency ringing on the SW pin

during discontinuous current mode operation. The ringing

of the resonant circuit formed by L and CSW (capacitance

on SW pin) is low energy, but can cause EMI radiation.

Output Disconnect

The LTC3528-2 is designed to allow true output discon-

nect by eliminating body diode conduction of the internal

P-channel MOSFET rectifier. This allows for VOUT to go to

zero volts during shutdown, drawing no current from the

input source. It also enables inrush current limiting at turn-

on, minimizing surge currents seen by the input supply.

Note that to obtain the advantages of output disconnect,

a Schottky diode cannot be connected between SW and

VOUT. The output disconnect feature also allows VOUT

to be forced above the programmed regulation voltage,

without any reverse current into the input power source.

Thermal Shutdown

If the die temperature exceeds 160°C, the LTC3528-2

enters thermal shutdown. All switches will be turned off

and the soft-start capacitor will be discharged. The device

will be enabled again when the die temperature drops by

approximately 15°C.

Burst Mode OPERATION

The LTC3528-2 will automatically enter Burst Mode op-

eration at light load current and return to fixed frequency

PWM mode when the load increases. Refer to the Typical

Performance Characteristics to see the output load Burst

Mode threshold vs VIN. The load at which Burst Mode

operation is entered can be changed by adjusting the

inductor value. Raising the inductor value will lower the

load current at which Burst Mode operation is entered.

In Burst Mode operation, the LTC3528-2 continues switch-

ing at a fixed frequency of 2MHz, using the same error

amplifier and loop compensation for peak current mode

control. This control method minimizes output transients

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