LT8705 datasheet(35/42 Pages) LINER

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LTC7813

7813f

For more information www.linear.com/LTC7813

applicaTions inForMaTion

OPTIONB: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.

LOOSELYREGULATEDOUTPUT(Pass-ThroughRegulator)

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% in forced continuous mode) and the buck

regulator is operating in dropout (with TG1 on at an effec-

tive 99%duty cycle). This makes the circuit very efficient,

especially at heavy loads, with extremely low input and

outputrippleandEMI.Notethatinthispass-throughmode,

the circuit does not benefit from the LTC7813’s ultralow

quiescent current of 33µA in Burst Mode operation 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 cre-

ate a filter that substantially reduces any input ripple and

results in very little conducted EMI on the input.

Table 3 summarizes some of the performance trade-offs

of these four potential ways to set the VMID regulation

voltage in an LTC7813 cascaded Boost+Buck regulator.

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