LT8335 datasheet(10/20 Pages) AD

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LT8338

Rev. 0

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APPLICATIONS INFORMATION

Programming VIN Turn-On and Turn-Off Thresholds

with the EN/UVLO Pin

The LT8338 is in shutdown when the EN/UVLO pin is

low. The falling threshold of the EN comparator is 1V,

with 50mV of hysteresis. The EN pin can be tied to VIN if

the shutdown feature is not used, or tied to a logic level

if shutdown control is required. The LT8338 draws very

low shutdown quiescent current (0.2µA typ) When EN/

UVLO is below 0.3V.

Adding a resistor divider from VIN to EN/UVLO programs

the LT8338 to regulate the output only when VIN is above

a desired voltage. Typically, this threshold, VIN(EN), is used

in situations where the input supply is current limited, or

has a relatively high source resistance. A switching reg-

ulator draws constant power from the source, so source

current increases as source voltage drops. This looks like

a negative resistance load to the source and can cause

the source to current limit or latch low under low source

voltage conditions. The VIN(EN) threshold prevents the

regulator from operating at source voltages where the

problems might occur. This threshold can be adjusted by

setting the values R3 and R4 (refer to the Block Diagram)

such that they satisfy Equation 1.

VIN,FALLING = 1V •

(R3+R4)

VIN,RISING = 40mV •

(R3+R4)

+ VIN,FALLING

(1)

When operating in Burst Mode operation for light load

applications, the current through the R3 and R4 resis-

tor network can easily be greater than the supply current

consumed by the LT8338. Therefore, R3 and R4 should

be large to minimize their effect on efficiency at low loads.

INTVCC Regulator

An internal low dropout (LDO) regulator produces the

2.6V supply from VIN that powers the drivers and the

internal bias circuitry. INTVCC can supply enough cur-

rent for the LT8338’s circuitry and must be bypassed

to ground with a minimum of 2.2μF low ESR ceramic

capacitor. Good bypassing is necessary to supply the

high transient currents required by the power MOSFET

gate drivers. Applications with high VIN voltage and high

switching frequency will increase die temperature due to

the higher power dissipation across the LDO. The INTVCC

falling threshold (to stop switching and reset soft-start)

is typically 2.2V, and the rising threshold is 2.3V. Do not

connect an external load to the INTVCC pin.

Achieving Ultralow Quiescent Current

When LT8338 is set for Burst Mode operation to enhance

efficiency at light loads, the minimum peak inductor cur-

rent is set to approximately 300mA even though VC node

indicates a lower value (refer to the Block Diagram). In

Burst Mode operation, the LT8338 delivers single pulses

of current to the output capacitor followed by sleep peri-

ods where the output power is supplied by the output

capacitor. That is, at light load condition, the LT8338

maintains the output regulation voltage by reducing the

switching frequency instead of reducing the inductor peak

current.

As the output load decreases, the frequency of single cur-

rent pulses decreases (see Figure 1) and the percentage of

time the LT8338 is in sleep mode increases, resulting in

much higher light load efficiency than typical converters.

By maximizing the time between pulses, the converter

quiescent current approaches 6μA for a typical application

when there is no output load. In addition, if high light load

efficiency is desired, a larger inductor value should be

chosen. See the Burst Mode Efficiency vs Inductor Value

curve in the Typical Performance Characteristics.

Figure 1. Burst Mode Frequency vs Load

OUT = 24V, L = 10µH

VIN = 3V

VIN = 6V

VIN =12V

VIN = 18V

VIN = 21V

LOAD CURRENT (mA)

100

0.001

0.01

0.1

8338 F01

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