LTC7801 datasheet(20/38 Pages) LINER

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LTC7801

7801f

For more information www.linear.com/LTC7801

Incontinuousmode,thesourcecurrentofthetopMOSFET

is a square wave of duty cycle (VOUT)/(VIN). To prevent

large voltage transients, a low ESR capacitor sized for the

maximumRMScurrentmustbeused.ThemaximumRMS

capacitor current is given by:

CIN Required IRMS ≈

IMAX

VIN

(VOUT)(VIN − VOUT)

[

]1/2

This formula has a maximum at VIN = 2VOUT, where IRMS

= IOUT/2. This simple worst-case condition is commonly

used for design because even significant deviations do not

offermuchrelief.Notethatcapacitormanufacturers’ripple

current ratings are often based on only 2000 hours of life.

This makes it advisable to further derate the capacitor, or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may be paralleled to meet

size or height requirements in the design. Due to the high

operating frequency of the LTC7801, ceramic capacitors

can also be used for CIN. Always consult the manufacturer

if there is any question.

A small (0.1μF to 1μF) bypass capacitor between the chip

VIN pin and ground, placed close to the LTC7801, is also

suggested. A small (≤10Ω) resistor placed between CIN

(C1) and the VIN pin provides further isolation.

The selection of COUT is driven by the effective series

resistance (ESR). Typically, once the ESR requirement

is satisfied, the capacitance is adequate for filtering. The

output ripple (∆VOUT) is approximated by:

∆VOUT ≈ ∆IL ESR+

8 • f • COUT













where f is the operating frequency, COUT is the output

capacitance and ∆IL is the ripple current in the inductor.

The output ripple is highest at maximum input voltage

since ∆IL increases with input voltage.

applicaTions inForMaTion

Setting Output Voltage

The LTC7801 output voltage is set by an external feedback

resistordividercarefullyplacedacrosstheoutput,asshown

in Figure 4. The regulated output voltage is determined by:

VOUT = 0.8V 1+













To improve the frequency response, a feedforward ca-

pacitor, CFF, may be used. Great care should be taken to

route the VFB line away from noise sources, such as the

inductor or the SW line.

7801 F04

LTC7801

VFB

CFF

VOUT

Figure 4. Setting Output Voltage

RUN Pin and Overvoltage/Undervoltage Lockout

The LTC7801 is enabled using the RUN pin. It has a rising

threshold of 1.2V with 80mV of hysteresis. Pulling the RUN

pin below 1.12V shuts down the main control loop. Pull-

ing it below 0.7V disables the controller and most internal

circuits, including the DRVCC and INTVCC LDOs. In this

state the LTC7801 draws only 10μA of quiescent current.

The RUN pin is high impedance below 3V and must be

externally pulled up/down or driven directly by logic. The

RUN pin can tolerate up to 150V (absolute maximum), so

it can be conveniently tied to VIN in always-on applications

where the controller is enabled continuously and never

shut down. Above 3V, the RUN pin has approximately a

15MΩ impedance to an internal 3V clamp.

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