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LTC4091

4091fa

For more information www.linear.com/LTC4091

APPLICATIONS INFORMATION

where fSW is the switching frequency, tON(MIN) is the

minimum switch-on time (~100ns), and tOFF(MIN) is the

minimumswitch-offtime(~150ns).Theseequationsshow

that duty cycle range increases when switching frequency

is decreased.

A good choice of switching frequency should allow ade-

quate input voltage range (see next section) and keep the

inductor and capacitor values small.

VIN Input Voltage Range

The maximum input voltage range for LTC4091 applica-

tions depends on the switching frequency, the Absolute

Maximum Ratings of the VIN and BOOST pins, and the

operating mode.

The high voltage switching regulator can operate from

input voltages up to 36V, and safely withstand input

voltages up to 60V. Note that while VIN > 38V (typical),

the LTC4091 will stop switching, allowing the output to

fall out of regulation.

While the high voltage regulator output is in start-up,

short-circuit, or other overload conditions, the switching

frequency should be chosen according to the following

discussion.

For safe operation at inputs up to 60V the switching

frequency must be low enough to satisfy VIN(MAX) ≥ 40V

according to the following equation. If lower VIN(MAX) is

desired, this equation can be used directly.

VIN(MAX) =

VHVOUT + VD

fSW • tON(MIN)

– VD + VSW

where VIN(MAX) is the maximum operating input voltage,

VHVOUT is the high voltage regulator output voltage, VD is

the catch diode drop (~0.5V), VSW is the internal switch

drop (~0.5V at max load), fSW is the switching frequency

(set by RT), and tON(MIN) is the minimum switch-on time

(~150ns). Note that a higher switching frequency will de-

press the maximum operating input voltage. Conversely,

a lower switching frequency will be necessary to achieve

safe operation at high input voltages.

If the output is in regulation and no short-circuit, start-

up, or overload events are expected, then input voltage

transients of up to 60V are acceptable regardless of the

switching frequency. In this mode, the LTC4091 may enter

pulse-skipping operation where some switching pulses

are skipped to maintain output regulation. In this mode

the output voltage ripple and inductor current ripple will

be higher than in normal operation.

The minimum input voltage is determined by either the

highvoltageregulator’sminimumoperatingvoltageof~6V

or by its maximum duty cycle (see equation in previous

section). The minimum input voltage due to duty cycle is:

VIN(MIN) =

VHVOUT + VD

1– fSW • tOFF(MIN)

– VD + VSW

where VIN(MIN)istheminimuminputvoltage,andtOFF(MIN)

is the minimum switch-off time (~150ns). Note that high-

er switching frequency will increase the minimum input

voltage. If a lower dropout voltage is desired, a lower

switching frequency should be used.

Inductor Selection and Maximum Output Current

AgoodchoicefortheinductorvalueisL=6.8µH(assuming

an 800kHz operating frequency). With this value the max-

imum load current will be ~2.4A. For details of maximum

outputcurrent,seeLinearTechnologyApplicationNote44.

The RMS current rating of the inductor must be greater

than the maximum load current and its saturation current

should be about 30% higher. Note that the maximum load

currentwillbeprogrammedchargecurrentplusthelargest

expected application load current. For robust operation in

faultconditions,thesaturationcurrentshouldbe~3.5A.To

keep efficiency high, the series resistance (DCR) should

be less than 0.1Ω. Table 1 lists several vendors and types

that are suitable.

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