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LTC7812

7812fc

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APPLICATIONS INFORMATION

switching, the efficiency is lower and the input ripple and

EMI, while predictable and still low, are higher than other

potential options.

OPTION B: VIN_MIN_OP < VOUT_REG < VMID_REG <

VIN_MAX_OP

This is similar to option A, but VMID_REG is set within the

normal operating input voltage range (e.g., VMID_REG =

14V). When VIN is well below VMID_REG, this option is

like Option A. But as VIN approaches VMID_REG, the boost

controller will gradually begin skipping cycles (even in

forced continuous mode) once it reaches minimum-on-

time. If VIN > VMID_REG, then VMID follows VIN. In this

region, OPTION B is more efficient than OPTION A since

the boost is not switching. But this is at the expense of

the cycle-skipping (non-constant frequency ripple) when

VIN is slightly below VMID_REG.

LOOSELY REGULATED OUTPUT (Pass-Through

Regulator)

In some applications, it is not critical that VOUT be tightly

regulated,butratherthatitremainswithinacertainvoltage

range. Suppose, in our example, that it is only important

thatVOUTbemaintainedwithinthenormalbatteryoperating

voltagerangeof10Vto16V.Wecanconsiderathirdoption:

OPTION C: VMID_REG = VIN_MIN_OP and VOUT_REG =

VIN_MAX_OP

Here we set VMID_REG = VIN_MIN_OP = 10V and VOUT_REG

= VIN_MAX_OP =16V. So the boost regulator only boosts

when VIN < 10V and the buck regulator only bucks when

VIN >16V. When VIN is between 10V to 16V, the circuit is

in a pass-through or wire mode where there is very little

switching. The boost regulator is not boosting (TG2 is on

100%inforcedcontinuousmode)andthebuckregulatoris

operating in dropout (with TG1 on at an effective 99%duty

cycle.) This makes the circuit very efficient, especially at

heavy loads, with extremely low input and output ripple

and EMI. Note that in this pass-through mode, the circuit

does not benefit from the LTC7812’s ultralow quiescent

current of 33µA in Burst Mode since the buck regulator

does not go to sleep because VOUT < VOUT_REG = 16V.

REGULATED OUTPUT VOLTAGE BELOW NORMAL

INPUT VOLTAGE OPERATING RANGE

In some applications, the desired output voltage might

be less than the minimum normal operating voltage, but

still higher than the worst-case minimum input voltage.

Consider our previous example, but instead suppose we

want VOUT= 5V. In this case, we can set our VMID_REG

such that:

OPTION D: VIN_MIN_OP > VMID_REG > VOUT_REG

So we might set VMID_REG just below 10V, so that the

boostregulatorneverswitcheswithinthenormaloperating

range and only needs to boost during the input voltage

dips below 10V. The buck controller always regulates the

VOUT to 5V, and the boost regulator’s inductor and VMID

capacitance create a filter that substantially reduces any

input ripple and results in very little conducted EMI on

the input.

Table 1 summarizes some of the performance trade-offs

of these four potential ways to set the VMID regulation

voltage in an LTC7812 cascaded Boost+Buck regulator.

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