Data Sheet

LT7826

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Rev. 0

12 of 19

MODE Pin and Standby Mode

With RUN pin voltage above 1.2V and INTVCC voltage above its UVLO threshold, LT7826 is ready for switching.

However, switching will not start until the MODE pin voltage is higher than 1.2V. The MODE pin is pulled low with an

internal 1.6MΩ resistor, which makes LT7826 default to not switching (standby) after power up. Connect the MODE

pin to INTVCC or VHIGH to enable the LT7826 switching operation after power-up. The MODE pin can handle a

maximum voltage of 12V.

The MODE pin can also be used for synchronization. It responds to a clock signal with a frequency between 100kHz

and 1MHz and an amplitude higher than 2.6V, which will be adopted as the switching frequency for full power

operation. Phase inverting is also included in the design, causing SWL to run antiphase to the clock at the MODE pin.

With this synchronization and phase inversion, interleave operation can be achieved when multiple LT7826s are

connected in parallel.

Pre-balance

With RUN pin voltage above 1.2V, INTVCC voltage above its UVLO threshold, and MODE pin voltage above 1.2V, the

LT7826 starts up and monitors the VHIGH and VLOW voltage continuously. To avoid surge current through the circuit,

LT7826 enters a pre-balance mode if VVLOW is lower than VVHIGH/2 – 265mV or higher than VVHIGH/2 + 265mV. In pre-

balance mode, the maximum internal power FET current is 485mA, and the switching frequency is 250kHz. When

VVLOW is within the window of VVHIGH/2 ± 265mV, LT7826 exits pre-balance mode and starts full power switching.

A function of enable/disable cycling over switching is included in the pre-balance mode operation in LT7826. With

this function, the pre-balance switching continues for 16.4ms to charge up the flying and output capacitors. If VVLOW

is still not close enough to VVHIGH/2 to activate full-power switching, LT7826 pauses switching for 49.1ms until its next

switching enabled interval. This enables/disables cycling to continue as long as needed, averting over temperature

if the flying or output capacitors are large, or if power pins are shorted to ground.

Over Current Protection

During steady-state operation, VLOW voltage drops linearly as load current increases. The overcurrent protection is

implemented by monitoring the voltage difference between VVLOW and VVHIGH/2. When the difference exceeds 320mV,

the LT7826 exits full power switching and reduces the maximum power Field-effect transistor (FET) current to 485mA.

When overcurrent condition is removed and VVLOW returns to within the window of VVHIGH ± 265mV, LT7826 enters full

power switching again. The function of enable/disable cycling over switching is also applied during overcurrent

condition, as shown by the waveforms in Figure 11.

FAULT Pin and Fault Response

The ������������������������������ pin is used for fault indication in LT7826, and is implemented with an open-drain structure. When LT7826

enters full power switching, the FAULT pin is released and may be pulled up externally to indicate the readiness of

output. The FAULT pin would be pulled low in other situations, such as pre-balance mode, overcurrent, or

overtemperature conditions. The overtemperature threshold in LT7826 is 175°C, above which all the switching stops.

The die temperature must be lower than 165°C to re-start switching in LT7826.