LTC7802EUFDM datasheet(20/34 Pages) AD

20 / 34 page

LTC7802

Rev. 0

For more information www.analog.com

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 LTC7802 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+

⎛

⎝⎜

⎞

⎠⎟

RUN Pins and Undervoltage Lockout

The two channels of the LTC7802 are enabled using

the RUN1, and RUN2 pins. The RUN pins have a rising

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 both

RUN pins below 0.7V disables the controllers and most

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

the LTC7802 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

the controller is 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.

The VIN UVLO thresholds can be computed as:

UVLO RISING = 1.2V 1

⎛

⎝⎜

⎞

⎠⎟

UVLO FALLING = 1.1V 1

⎛

⎝⎜

⎞

⎠⎟

The current that flows through the R1-R2 divider directly

adds to the shutdown, sleep, and active current of the

LTC7802, and care should be taken to minimize the

impact of this current on the overall efficiency of the

application circuit. Resistor values in the MΩ range may

be required to keep the impact on quiescent shutdown

and sleep currents low.

APPLICATIONS INFORMATION

Figure 4. Setting Output Voltage

7802 F04

1/2 LTC7802

VFB

CFF

VOUT

Figure 5. Using the RUN Pins As a UVLD

7802 F06

1/2 LTC7802

RUN

VIN

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

SW trace. To improve frequency response, a feedforward

capacitor (CFF) may be used.

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.

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