LTC3370 datasheet(27/44 Pages) LINER

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LTC3372

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

CIN and COUT Selection

The selection of CIN is usually based off the worst-case

RMSinputcurrent.Thehighest(VOUT)(IOUT)productneeds

to be used in the formula to determine the maximum RMS

capacitor current requirement.

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 offer much relief. Note that capacitor manufacturers’

ripple current ratings are often based on only 2000 hours

of life. This makes it advisable to further derate the capaci-

tor, or to choose a capacitor rated at a higher temperature

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

high operating frequency, 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 IC, 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:

ΔVOUT ≈ ΔIL 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.

Setting Output Voltage (HV Controller)

The HV controller output voltage is set by the VOUTPRG

pin through an internal resistor voltage divider. When

the VOUTPRG pin is tied to INTVCC, the output voltage is

regulated to 5V. When tied to ground, the output voltage

is regulated to 3.3V.

RUN Pin

The HV controller is enabled using the RUN pin. It has a

rising threshold of 1.2V with 50mV of hysteresis. Pulling

the RUN pin below 1.15V shuts down the main control

loop.Pullingitbelow0.7Vdisablesthecontrollerandmost

internal circuits, including the INTVCC LDOs.

Releasing the RUN pin allows a 0.5μA internal current to

pull up the pin to enable the controller. The RUN pin may

be externally pulled up to up to 60V (65V Abs Max).

The RUN pin can be implemented as a UVLO by connect-

ing it to the output of an external resistor divider network

off VIN, as shown in Figure 4.

TherisingandfallingUVLOthresholdsarecalculatedusing

the RUN pin thresholds:

VUVLO(RISING) = 1.2V 1+

⎛

⎝

⎜

⎞

⎠

⎟

VUVLO(FALLING) = 1.15V 1+

⎛

⎝

⎜

⎞

⎠

⎟

The resistor values should be carefully chosen such that

the absolute maximum ratings of the RUN pin do not get

violated over the entire VIN voltage range.

LTC3372

RUN

VIN

3372 F04

Figure 4. Using the RUN Pin as a UVLO

High Voltage Buck Controller

LTC3370 Datasheet(PDF) 27 Page - Linear Technology

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