LT8601 datasheet(14/26 Pages) LINER

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LT8601

8601fa

For more information www.linear.com/LT8601

applicaTions inForMaTion

The internal oscillator of the LT8601 can be synchronized

to an external 250kHz to 2.2MHz clock signal on the SYNC

pin.

VIN Voltage Range

The LT8601’s minimum operating voltage is 3V. To pro-

gram a higher minimum operating voltage, use a resis-

tor divider between the VIN pin and the EN/UVLO pin. The

EN/UVLO threshold is 1.2V. The EN/UVLO pin has 50mV

of hysteresis to prevent glitches from falsely disabling

the LT8601.

The UVLO circuit is shown in Figure 3, Reverse Protection

Diodes. The calculation for the lockout voltage is:

VIN(UVLO) =

RUV1+RUV2

RUV2

•1.2V

PVIN Voltage Range

Each switching regulator channel operates from its own

PVIN pin (PVIN1 to PVIN3). The PVIN pin can be connected

to either an independent voltage supply or a high voltage

channel output. The PVIN1 and PVIN2 voltage range is 3.0V

to 42V. The PVIN3 voltage range is 2.6V to 5.5V.

The minimum PVIN voltage to regulate output voltage at

full frequency is:

PVINx(MIN) =

VOUTx

DCMAX

where DCMAX is the maximum duty cycle (refer to

Switching Frequency section) for that channel. If PVIN

is below the calculated minimum voltage, the channel

starts to skip switch off-cycles. At low input voltages,

the part will turn on the top switch for longer than a full

switch cycle in order to extend the effective duty cycle.

When the part is extending the effective duty cycle, the

switching frequency will drop to one half (or less) of the

programmed frequency.

The maximum PVIN voltage to regulate output voltage at

full frequency is:

PVINx(MAX) =

VOUTx

DCMIN

where DCMIN is the minimum duty cycle (refer to

Switching Frequency section) for that channel. If PVIN

is above the calculated maximum voltage, the channel

starts to skip switch on-cycles (pulse-skipping). In this

case, the channel switching frequency will no longer be

the programmed frequency. The output will continue to

regulate, but the peak inductor current and output ripple

will increase significantly.

Inductor Selection

Inductor selection involves inductance, saturation current,

series resistance (DCR) and magnetic loss.

A good starting point for the inductance values are:

Lx =Kx •

VOUTx

PVINx

• PVINx – VOUTx

where fS is the switching frequency in MHz, Lx is in µH,

VOUTx is the channel output voltage and K1 = 1.7, K2 = 1.0

and K3 = 1.4.

Once the inductance is selected, the inductor current

ripple and peak current can be calculated:

ΔILx =

VOUTx

Lx • fS

• 1–

VOUTx

PVINx(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

ILx(PEAK) =IOUTx(MAX)+

ΔILx

To guarantee sufficient output current, peak inductor cur-

rent must be lower than the switch current limit (ILIM).

To keep the efficiency high, the inductor series resistance

(DCR) should be as small as possible (must be < 0.1Ω

channels 1 and 3; < 0.06 Ω channel 2), and the core mate-

rial should be intended for the chosen switching frequency.

Table 2 lists several vendors and suitable inductor series.

Table 2. Inductor Vendors

VENDOR

SERIES

WEBSITE

TDK

SLF, VLC, VLF

www.tdk.com

Sumida

CDRH, CDR, CDMC

www.sumida.com

Coilcraft

XAL, XFL, MSS

www.coilcraft.com

NIC

NPIM, NPIS

www.niccomp.com

Würth

TPC, SPC, PD, PDF, PD3

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