LTC7891 datasheet(24/36 Pages) AD

24 / 36 page

Data Sheet

LTC7891

APPLICATIONS INFORMATION

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Rev. 0 | 24 of 36

Figure 44. Setting Adjustable Output Voltage

Figure 45. Setting Fixed 12 V or 5 V Voltage

RUN PIN AND UNDERVOLTAGE LOCKOUT

The LTC7891 is enabled using the RUN pin. The RUN pin has a

rising threshold of 1.2 V with 100 mV of hysteresis. Pulling the RUN

pin below 1.08 V shuts down the main control loop and resets the

soft start. Pulling the RUN pin below 0.7 V disables the controller

and most internal circuits, including the INTVCC LDO regulators. In

this state, the LTC7891 draws only ≈1 μA of quiescent current.

The RUN pin is high impedance, must be externally pulled up or

pulled down, and is driven directly by logic. The RUN pin can

tolerate up to 100 V (the absolute maximum). Therefore, the pin

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 pin.

The RUN pin can also be configured as a precise UVLO on the

input supply with a resistor divider from VIN to GND, as shown in

Figure 46.

Figure 46. Using the RUN Pin as a UVLO

The VIN UVLO thresholds can be computed by Equation 17 and

Equation 18, as follows:

UVLO Rising=1.2V1+R1R2

(17)

UVLO Falling=1.08V1+R1R2

(18)

The current that flows through the R1 and R2 divider adds to the

shutdown, sleep, and active current of the LTC7891. Take care to

minimize the impact of this current on the overall efficiency of the

application circuit. Resistor values in the MΩ range can be required

to keep the impact on quiescent shutdown and sleep currents low.

SOFT START AND TRACKING (TRACK/SS PIN)

The startup of VOUT is controlled by the voltage on the TRACK/SS

pin. When the voltage on the TRACK/SS pin is less than the inter-

nal 0.8 V reference, the LTC7891 regulates the VFB pin voltage to

the voltage on the TRACK/SS pin instead of the internal reference.

The TRACK/SS pin can be used to program an external soft start

function, or to allow VOUT to track another supply during startup.

Soft start is enabled by connecting a capacitor from the TRACK/SS

pin to GND. An internal 12 μA current source charges the capacitor,

providing a linear ramping voltage at the TRACK/SS pin. The

LTC7891 regulates its feedback voltage (and hence VOUT) accord-

ing to the voltage on the TRACK/SS pin, allowing VOUT to rise

smoothly from 0 V to its final regulated value. For a desired soft

start time (tSS), select a soft start capacitor (CSS) = tSS × 15 nF/ms.

Alternatively, the TRACK/SS pin can be used to track another

supply during startup, as shown qualitatively in Figure 47 and

Figure 48. To track another supply, connect a resistor divider from

the leader supply (VX) to the TRACK/SS pin of the follower supply

(VOUT), as shown in Figure 49. During startup, VOUT tracks VX,

according to the ratio set by the resistor divider in Equation 19:

VXVOUT

= RARTRACKA×RTRACKA+RTRACKB

RA+RB

(19)

Set RTRACKA = RA and RTRACKB = RB for coincident tracking (VOUT =

VX during startup).

Figure 47. Coincident Tracking

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