MAX17075(2008) View Datasheet(PDF) - Maxim Integrated

Part Name

Description

MFG CO.

MAX17075
(Rev.:2008)

Boost Regulator with Integrated Charge Pumps, Switch Control, and High-Current Op Amp

Maxim Integrated 

Boost Regulator with Integrated Charge Pumps,

Switch Control, and High-Current Op Amp

The inductor’s saturation current rating and the

MAX17075’s LX current limit should exceed IAVDD_PEAK,

and the inductor’s DC current rating should exceed

IIN(DC,MAX). For good efficiency, choose an inductor with

less than 0.1Ω series resistance.

Considering the typical operating circuit, the maximum

load current (IAVDD(MAX)) is 500mA with a 13V output

and a typical input voltage of 5V. Choosing an LIR of 0.5

and estimating efficiency of 85% at this operating point:

LAVDD

=

⎛ 5V ⎞ 2

⎝⎜ 13V ⎠⎟

⎛

⎝⎜

13V − 5V ⎞

0.5A × 1.2MHz ⎠⎟

⎛

⎝⎜

0.85 ⎞

0.5 ⎠⎟

≈

3.35μH

Using the circuit’s minimum input voltage (2.5V) and

estimating efficiency of 80% at that operating point:

IIN(DC,MAX)

=

0.5A × 13V

2.5V × 0.8

≈

3.25A

The ripple current and the peak current are:

IRIPPLE

=

2.5V × (13V − 2.5V)

3.3µH × 13V × 1.2MHz

≈

0.51A

IPEAK

=

3.25A

+

0.51A

2

≈

3.51A

Output Capacitor Selection

The total output voltage ripple has two components: the

capacitive ripple caused by the charging and discharg-

ing of the output capacitance, and the ohmic ripple due

to the capacitor’s equivalent series resistance (ESR):

VAVDD _RIPPLE = VAVDD _RIPPLE(C) + VAVDD _RIPPLE(ESR)

VAVDD _RIPPLE(C)

≈

IAVDD

CAVDD

⎛

⎝⎜

VAVDD − VIN ⎞

VAVDDfSW ⎠⎟

,

and

VAVDD _RIPPLE(ESR) ≈ IPEAKRESR _ AVDD

where IPEAK is the peak inductor current (see the

Inductor Selection section). For ceramic capacitors, the

output voltage ripple is typically dominated by

VAVDD_RIPPLE(C). The voltage rating and temperature

characteristics of the output capacitor must also be

considered.

Input-Capacitor Selection

The input capacitor (CIN) reduces the current peaks

drawn from the input supply and reduces noise injec-

tion into the IC. Two 10µF ceramic capacitors are used

in the typical operating circuit (Figure 1) because of the

high source impedance seen in typical lab setups.

Actual applications usually have much lower source

impedance since the step-up regulator often runs

directly from the output of another regulated supply.

Typically, CIN can be reduced below the values used in

the typical operating circuit. Ensure a low-noise supply

at VCC by using adequate CIN. Alternately, greater volt-

age variation can be tolerated on CIN if VCC is decou-

pled from CIN using an RC lowpass filter (see R1 and

C5 in Figure 1).

Rectifier Diode

The MAX17075’s high switching frequency demands a

high-speed rectifier. Schottky diodes are recommend-

ed for most applications because of their fast recovery

time and low forward voltage. In general, a 2A Schottky

diode complements the internal MOSFET well.

Output Voltage Selection

The output voltage of the step-up regulator can be

adjusted by connecting a resistive voltage-divider from

the output (VAVDD) to ground with the center tap con-

nected to FB (see Figure 1). Select R9 in the 10kΩ to

50kΩ range. Calculate R8 with the following equation:

R8

=

R9

×

⎛

⎝⎜

VAVDD

VFB

−

⎞

1⎠⎟

where VFB, the step-up regulator’s feedback set point,

is 1.25V. Place R8 and R9 close to the IC.

Loop Compensation

Choose RCOMP (R10 in Figure 1) to set the high-fre-

quency integrator gain for fast-transient response.

Choose CCOMP (C12 in Figure 1) to set the integrator

zero to maintain loop stability.

For low-ESR output capacitors, use the following equa-

tions to obtain stable performance and good transient

response:

RCOMP

≈

312.5 × VIN

LAVDD

× VAVDD × CAVDD

× IAVDD(MAX)

CCOMP ≈

VAVDD × CAVDD

10 × IAVDD(MAX)RCOMP

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