LT8330HS6 データシートの表示（PDF） - Linear Technology

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LT8330HS6

Low IQ Boost/SEPIC/ Inverting Converter with 1A, 60V Switch

Linear Technology 

LT8330

Applications Information

Setting the Output Voltage

The output voltage is programmed with a resistor divider

from the output to the FBX pin. Choose the resistor values

for a positive output voltage according to:

R1 = R2 • (VOUT/1.60V – 1)

Choose the resistor values for a negative output voltage

according to:

R1 = R2 • (|VOUT|/0.80V – 1)

The locations of R1 and R2 are shown in the Block Dia-

gram. 1% resistors are recommended to maintain output

voltage accuracy.

Higher-value FBX divider resistors result in the lowest input

quiescent current and highest light-load efficiency. FBX

divider resistors R1 and R2 are usually in the range from

25k to 1M. Most applications use a phase-lead capacitor

from VOUT to FBX in combination with high-value FBX

divider resistors (see Compensation in the Applications

Information section).

Soft-Start

The LT8330 contains several features to limit peak switch

currents and output voltage (VOUT) overshoot during

start-up or recovery from a fault condition. The primary

purpose of these features is to prevent damage to external

components or the load.

High peak switch currents during start-up may occur in

switching regulators. Since VOUT is far from its final value,

the feedback loop is saturated and the regulator tries to

charge the output capacitor as quickly as possible, resulting

in large peak currents. A large surge current may cause

inductor saturation or power switch failure.

The LT8330 addresses this mechanism with an internal

soft-start function. As shown in the Block Diagram, the

soft-start function controls the ramp of the power switch

current by controlling the ramp of VC through M2. This

allows the output capacitor to be charged gradually toward

its final value while limiting the start-up peak currents.

Figure 3 shows the output voltage and supply current for

the first page Typical Application. It can be seen that both

the output voltage and supply current come up gradually.

IL

500mA/DIV

VOUT

20V/DIV

500µs/DIV

8330 F03

Figure 3. Soft-Start Waveforms

INTVCC undervoltage (INTVCC < 2.5V) and/or thermal

lockout (TJ > 170°C) will immediately prevent switching,

will reset the internal soft-start function and will pull down

VC. Once all faults are removed, the LT8330 will soft-start

VC and hence inductor peak current.

Frequency Foldback

During start-up or fault conditions in which VOUT is very

low, extremely small duty cycles may be required to

maintain control of inductor peak current. The minimum

on-time limitation of the power switch might prevent these

low duty cycles from being achievable. In this scenario

inductor current rise will exceed inductor current fall during

each cycle, causing inductor current to ‘walk up’ beyond

the switch current limit. The LT8330 provides protection

from this by folding back switching frequency whenever

FBX pin is close to GND (low VOUT levels). This frequency

foldback provides a larger switch-off time, allowing inductor

current to fall enough each cycle (see Normalized Switch-

ing Frequency vs FBX Voltage in the Typical Performance

Characteristics section).

Thermal Lockout

If the LT8330 die temperature reaches 170°C (typical),

the part will stop switching and go into thermal lockout.

When the die temperature has dropped by 5°C (nominal),

the part will resume switching with a soft-started inductor

peak current.

8330fa

10

For more information www.linear.com/LT8330

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