LTC7803 datasheet(22/36 Pages) AD

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LTC7819

Rev. 0

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

chip VIN pin and ground, placed close to the LTC7819, is

also suggested. An optional 1Ω to 10Ω resistor placed

between CIN and the VIN pin provides further isolation

from a noisy input supply.

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 (ΔVOUT) is approximated by:

ΔVOUT ≈ ΔIL ESR +

8fCOUT

⎛

⎝

⎜

⎞

⎠

⎟

where f is the operating frequency, COUT is the output

capacitance and ΔIL is the ripple current in the inductor.

The output ripple is highest at maximum input voltage

since ΔIL increases with input voltage.

Setting the Output Voltage

The LTC7819 output voltages are each set by an external

feedback resistor divider carefully placed across the out-

put, as shown in Figure 4. The regulated output voltage

is determined by:

VOUT = 0.8V 1+

⎛

⎝

⎜

⎞

⎠

⎟

Place resistors RA and RB very close to the VFB pin to

minimize PCB trace length and noise on the sensitive VFB

node. Great care should be taken to route the VFB trace

away from noise sources, such as the inductor or the

Channel 1 can be programmed to a fixed 5V or 3.3V out-

put through control of the VPRG1 pin, which optionally

connects an internal divider to the VFB1 pin. Tying VPRG1

to INTVCC or GND programs VOUT1 to 5V or 3.3V, respec-

tively. Floating VPRG1 sets VOUT1 to adjustable output

mode using external resistors. When channel 1 is con-

figured for a fixed output voltage setting, directly connect

VFB1 to the output VOUT1.

For applications with multiple output voltage levels, select

channel 1 to be the lowest output voltage that is greater

than 3.2V. When the SENSE1– pin (connected to VOUT1)

is above 3.2V, it biases some internal circuitry instead of

VIN, thereby increasing light load Burst Mode efficiency.

Similarly, connect EXTVCC to the lowest output voltage that

is greater than the 4.8V maximum EXTVCC rising switch-

over threshold. EXTVCC then supplies the high current gate

drivers and relieves additional quiescent current from VIN,

further reducing the VIN pin current to ≈1μA in sleep.

RUN Pins and Undervoltage Lockout

The three channels of the LTC7819 are enabled using the

RUN1, RUN2, and RUN3 pins. The RUN pins have a ris-

ing threshold of 1.2V with 100mV of hysteresis. Pulling

a RUN pin below 1.1V shuts down the main control loop

and resets the soft-start for that channel. Pulling all three

RUN pins below 0.7V disables the controllers and most

internal circuits, including the INTVCC LDOs. In this state,

the LTC7819 draws only ≈1.5μA of quiescent current.

The RUN pins are high impedance and must be externally

pulled up/down or driven directly by logic. Each RUN pin

can tolerate up to 40V (absolute maximum), so it can be

conveniently tied to VIN in always-on applications where

one or more controllers are enabled continuously and

never shut down. Do not float the RUN pins.

The RUN pins can also be configured as precise under-

voltage lockouts (UVLOs) on the input supply with a resis-

tor divider from VIN to ground, as shown in Figure 5.

APPLICATIONS INFORMATION

SW trace. To improve frequency response, a feedforward

capacitor (CFF) may be used.

Figure 4. Setting Output Voltage

7819 F04

1/3 LTC7819

VFB

CFF

VOUT

Figure 5. Using the RUN Pins As a UVLD

7819 F05

1/3 LTC7819

RUN

VIN

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