3800EFE データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

メーカー

3800EFE

High-Voltage Synchronous Current Mode Step-Down Controller

Linear Technology 

LT3800

APPLICATIO S I FOR ATIO

Burst Mode

The LT3800 employs low current Burst Mode functionality

to maximize efficiency during no load and low load condi-

tions. Burst Mode operation is enabled by shorting the

BURST_EN pin to SGND. Burst Mode operation can be

disabled by shorting BURST_EN to either VFB or VCC.

When the required switch current, sensed via the VC pin

voltage, is below 15% of maximum, the Burst Mode

operation is employed and that level of sense current is

latched onto the IC control path. If the output load requires

less than this latched current level, the converter will

overdrive the output slightly during each switch cycle.

This overdrive condition is sensed internally and forces

the voltage on the VC pin to continue to drop. When the

voltage on VC drops 150mV below the 15% load level,

switching is disabled and the LT3800 shuts down most of

its internal circuitry, reducing total quiescent current to

100µA. When the converter output begins to fall, the VC pin

voltage begins to climb. When the voltage on the VC pin

climbs back to the 15% load level, the IC returns to normal

operation and switching resumes. An internal clamp on

the VC pin is set at 100mV below the switch disable

threshold, which limits the negative excursion of the pin

voltage, minimizing the converter output ripple during

Burst Mode operation.

During Burst Mode operation, VIN pin current is 20µA and

VCC current is reduced to 80µA. If no external drive is

provided for VCC, all VCC bias currents originate from the

VIN pin, giving a total VIN current of 100µA. Burst current

can be reduced further when VCC is driven using an output

derived source, as the VCC component of VIN current is

then reduced by the converter buck ratio.

Reverse-Current Inhibit

The LT3800 contains a reverse-current inhibit feature to

maximize efficiency during light load conditions. This

mode of operation allows discontinuous operation, and is

sometimes referred to as “pulse-skipping” mode. Refer to

Figure 2.

This feature is enabled with Burst Mode operation, and can

also be enabled while Burst Mode operation is disabled by

shorting the BURST_EN pin to VFB.

When reverse-current inhibit is enabled, the LT3800 sense

amplifier detects inductor currents approaching zero and

disables the synchronous switch for the remainder of the

switch cycle. If the inductor current is allowed to go

negative before the synchronous switch is disabled, the

switch node could inductively kick positive with a high

dv/dt. The LT3800 prevents this by incorporating a 10mV

positive offset at the sense inputs.

PULSE SKIP MODE

IL

IL FORCED CONTINUOUS

DECREASING

LOAD

CURRENT

3800 F02

Figure 2. Inductor Current vs Mode

3800fb

10

Share Link: