LT8335 datasheet(9/20 Pages) LINER

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LT8335

8335f

For more information www.linear.com/LT8335

APPLICATIONS INFORMATION

While in Burst Mode operation, the current limit of the

switch is approximately 500mA, resulting in the output

voltage ripple shown in Figure 2. Increasing the output

capacitance will decrease the output ripple proportionally.

As the output load ramps upward from zero the switch-

ing frequency will increase but only up to the fixed 2MHz

defined by the internal oscillator as shown in Figure 1. The

output load at which the LT8335 reaches the fixed 2MHz

frequency varies based on input voltage, output voltage,

and inductor choice.

Figure 2. Burst Mode Operation

PROGRAMMING INPUT TURN-ON AND TURN-OFF

THRESHOLDS WITH EN/UVLO PIN

The EN/UVLO pin voltage controls whether the LT8335 is

enabled or is in a shutdown state. A 1.6V reference and a

comparatorA6withbuilt-inhysteresis(typical80mV)allow

the user to accurately program the system input voltage

at which the IC turns on and off (see the Block Diagram).

The typical input falling and rising threshold voltages can

be calculated by the following equations:

VIN(FALLING,UVLO(–)) =1.60•

R3+R4

VIN(FALLING,UVLO(–)) =1.68•

R3+R4

VIN current is reduced below 1µA when the EN/UVLO pin

voltage is less than 0.2V. The EN/UVLO pin can be con-

nected directly to the input supply VIN for always-enabled

operation. A logic input can also control the EN/UVLO pin.

When operating in Burst Mode operation for light load

currents, the current through the R3 and R4 network can

easily be greater than the supply current consumed by the

LT8335. Therefore, R3 and R4 should be large enough to

minimize their effect on efficiency at light loads.

INTVCC REGULATOR

A low dropout (LDO) linear regulator, supplied from VIN,

produces a 3V supply at the INTVCC pin. A minimum 1µF

low ESR ceramic capacitor must be used to bypass the

INTVCC pin to ground to supply the high transient currents

required by the internal power MOSFET gate driver.

No additional components or loading is allowed on this

pin. The INTVCC rising threshold (to allow soft start and

switching) is typically 2.6V. The INTVCC falling threshold

(to stop switching and reset soft start) is typically 2.5V.

DUTY CYCLE CONSIDERATION

The LT8335 minimum on-time, minimum off-time and

switching frequency (fOSC) define the allowable minimum

and maximum duty cycles of the converter (see Minimum

On-Time, Minimum Off-Time, and Switching Frequency

in the Electrical Characteristics table).

Minimum Allowable Duty Cycle =

Minimum On-Time(MAX) • fOSC(MAX)

Maximum Allowable Duty Cycle =

1 – Minimum Off-Time(MAX) • fOSC(MAX)

The required switch duty cycle range for a Boost converter

operating in continuous conduction mode (CCM) can be

calculated as:

DMIN =1–

VIN(MAX)

VOUT +VD

DMAX =1–

VIN(MIN)

VOUT +VD

where VD is the diode forward voltage drop. If the above

duty cycle calculations for a given application violate the

minimum and/or maximum allowed duty cycles for the

LT8335, operation in discontinuous conduction mode

(DCM) might provide a solution. For the same VIN and

VOUT levels, operation in DCM does not demand as low a

duty cycle as in CCM. DCM also allows higher duty cycle

operation than CCM. The additional advantage of DCM is

5µs/DIV

VOUT

10mV/DIV

500mA/DIV

8335 F02

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