LTC3535 datasheet(10/16 Pages) LINER

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LTC3535

3535fa

the test mode being engaged. Refer to Figure 1 for two

possible implementations.

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 30mA. 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-ring circuit connects a resistor across the in-

ductor to prevent high frequency ringing on the SW pin

during discontinuous current mode operation. Although

the ringing of the resonant circuit formed by L and CSW

(capacitance on SW pin) is low energy, it can cause EMI

radiation.

Output Disconnect

The LTC3535 is designed to allow true output disconnect

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 allows for inrush current limiting

at turn-on, minimizing surge currents seen by the input

supply. Note that to obtain the advantages of output dis-

connect, there must not be an external Schottky diode

connected between SW and VOUT. The output disconnect

feature also allows VOUT to be pulled high, without any

reverse current into a battery connected to VIN.

Thermal Shutdown

If the die temperature exceeds 160°C, the LTC3535 will

go into thermal shutdown. All switches will be off and

the soft-start capacitor will be discharged. The device

will be enabled again when the die temperature drops by

about 15°C.

OPERATION (Refer to Block Diagram)

3535 F01

R > (VCNTRL/VIN + 0.4) – 1)M

±30%

LTC3535

SHDN

VCNTRL

ZETEX

ZC2811E

±30%

LTC3535

SHDN

VIN

Figure 1. Recommended Shutdown Circuits When

Driving SHDN Above VIN

Error Amplifier

The positive input of the transconductance error amplifier

is internally connected to the 1.195V reference and the

negative input is connected to FB. Clamps limit the mini-

mumandmaximumerrorampoutputvoltageforimproved

large-signal transient response. Power converter control

loop compensation is provided internally. An external

resistive voltage divider from VOUT to ground programs

the output voltage via FB from 1.5V to 5.25V.

OUT =+









1 195

.•

Current Sensing

Losslesscurrentsensingconvertsthepeakcurrentsignalof

theN-channelMOSFETswitchintoavoltagethatissummed

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

MOSFET switch once its threshold is reached. The cur-

rent limit comparator delay to output is typically 60ns.

Peak switch current is limited to approximately 750mA,

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