ADP1621ARMZ-R7 datasheet(23/32 Pages) AD

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ADP1621

Rev. A | Page 23 of 32

BOOTSTRAPPED BOOST CONVERTER

The inputs of the ADP1621 can be driven from the step-up

converter output voltage to improve efficiency for low input

voltages. For low input voltages, bootstrapped operation improves

efficiency with heavy loads by increasing the available gate drive

voltage, thus reducing the on resistance of the MOSFET. However,

because the internal circuitry is driven from IN, the ADP1621

quiescent current and gate drive current supplied from the input

increases due to the step-up ratio and the conversion efficiency loss.

The circuit shown in Figure 1 shows a bootstrapped boost con-

verter, where VIN = 3.3 V and VOUT = 5 V. To ensure that the circuit

starts, make sure that the input voltage minus the forward-voltage

drop of the diode is greater than the UVLO voltage and the gate

threshold voltage of the MOFSET. In this example, the MOSFET

has a gate threshold voltage of 2.5 V. The regulator shown in

Figure 1 is very similar to that shown in Figure 33, which is a

standard boost without bootstrapping. Because the same MOSFET

and inductor are used in both circuits and the input and output

conditions are the same, the compensation components remain

unchanged.

Figure 34 shows a bootstrapped application circuit for output

voltages greater than 5.5 V. In this case, the output is 12 V.

Notice that a resistor, R3, of 700 Ω is placed between VOUT and

the IN and PIN pins to limit the input currents because the IN

and PIN pins are regulated to 5.5 V. A diode, D2, is placed between

VIN and the IN/PIN pins to supply the necessary quiescent current

to start the ADP1621. Once the ADP1621 starts and the output

voltage reaches 12 V, the quiescent current stops flowing

through D2 and is supplied by the output. Keep in mind that the

dynamic supply current to PIN increases as the switching fre-

quency increases because more gate drive is needed for a higher

switching frequency. Therefore, R3 needs to be set appropriately.

The PIN supply current can be approximated by

(45)

where IPIN is the PIN supply current, fSW is the switching frequency,

and QG is the gate charge of a particular MOSFET.

An alternative implementation to Figure 34 is shown in Figure 35,

where an NPN transistor is used to supply the necessary current

to the input PIN at various loads, but the gate drive voltage is

limited to approximately 4.8 V (one diode drop below the

voltage at IN). Signal Diodes D2 and D3 help to provide the

necessary quiescent current to start the ADP1621. Once the

ADP1621 starts, the current stops flowing through these two

diodes because the voltages at PIN and IN are approximately

4.8 V and 5.5 V, respectively. One advantage of this technique

is that Q1 provides enough current to the gate driver at any

switching frequency with a wide range of MOSFETs that have

different gate charge specifications.

Notice that the output capacitor, COUT2 in Figure 34 and Figure 35,

is a large aluminum electrolytic capacitor, both in physical size

and capacitance. Such capacitors are very cheap relative to

ceramic capacitors (such as Sanyo POSCAP) or aluminum

polymer capacitors. The ADP1621 can work with a wide range

of capacitor types.

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