LT3640EUFD datasheet(13/24 Pages) LINER

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LT3640

3640p

Figure 1. Lower Switching Frequency Occurs in High

Voltage Channel When Required On-Time Is Below 50ns

The maximum VIN should not exceed the absolute maxi-

mum rating. For ﬁxed frequency operation, the maximum

VIN is:

IN MAX

OUT

MIN

DCE

() =

⎛

⎝⎜

⎞

⎠⎟

−+

Note that the high voltage buck will still regulate at an input

voltage that exceeds VIN(MAX) (up to 36V). However, the

switching frequency will be lowered to satisfy the equa-

tion (Figure 1).

Once the input voltage reaches 36V, an internal overvoltage

lockout (OVLO) circuit is triggered to disable switching ac-

tion (Figure 2). Without switching, the LT3640 can sustain

VIN voltage transients up to 55V for one second.

VIN2 Voltage Range

The low voltage synchronous channel operates from

the VIN2 pin. The VIN2 pin can be connected to either an

independent voltage supply or the high voltage channel

output for a two-stage power regulator.

In either conﬁguration, if the high voltage channel is over-

loaded and pulled out of regulation, the low voltage channel

will be disabled. The SS2 pin will be discharged as well.

The minimum VIN2 voltage to regulate output voltage is:

IN MIN

OUT

MAX

() ≈

Where DCMAX is the maximum duty cycle (refer to

the Switching Frequency section). If VIN2 is below the

calculated minimum voltage, the output will fall out of

regulation.

The maximum VIN2 for ﬁxed frequency operation is:

IN MAX

OUT

MIN

() ≈

Where DCMIN is the minimum duty cycle (refer to the

Switching Frequency section). For voltage that exceeds

VIN2(MAX) (up to 5.5V), the low voltage channel exhi-

bits pulse-skipping behavior, and the output ripple will

increase.

Inductor Selection

Inductor selection involves inductance, saturation current,

series resistance (DCR) and magnetic loss. The inductor

current ripple determines the inductance. A reasonable cur-

rent ripple is around 30% of the maximum load current:

ΔIL = 0.3 • IOUT(MAX)

where IOUT(MAX) is the maximum load current. To guaran-

tee sufﬁcient output current, peak inductor current must

be lower than the switch current limit (ILIM). The peak

inductor current is:

L PEAK

OUT MAX

()

(

)

where IL(PEAK) is the peak inductor current. For the high

voltage channel, the top switch current limit is at least

2.4A at low duty cycles and decreases linearly to 1.8A at

DC = 0.8. Be sure to pick an inductor ripple current that

provides sufﬁcient maximum load current IOUT(MAX). Once

Figure 2. VIN Overvoltage Lockout

APPLICATIONS INFORMATION

200ns/DIV

SW1

10V/DIV

IL1

0.5A/DIV

3640 F01

VIN = 30V

VOUT1 = 3.3V/0.2A

RT SET = 2MHz

10μs/DIV

VIN

20V/DIV

55VPK, 40V, 15V

IL1

2A/DIV

3640 F02

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