______________________________________________________________________________________ 13

76V, 300mW Boost Converter and Current

Monitor for APD Bias Applications

Diode Selection

The MAX15059’s high switching frequency demands a

high-speed rectifier. Schottky diodes are recommended

for most applications because of their fast recovery time

and low forward-voltage drop. Ensure that the diode’s

peak current rating is greater than the peak inductor cur-

rent. Also, the diode breakdown voltage must be greater

Output Filter Capacitor Selection

For most applications, use a small output capacitor of

0.1FF or greater. To achieve low output ripple, a capaci-

tor with low ESR, low ESL, and high capacitance value

should be selected. If tantalum or electrolytic capacitors

are used to achieve high capacitance values, always

add a smaller ceramic capacitor in parallel to bypass the

high-frequency components of the diode current. The

higher ESR and ESL of electrolytic capacitors increase

the output ripple and peak-to-peak transient voltage.

Assuming the contribution from the ESR and capacitor

discharge equals 50% (proportions may vary), calculate

the output capacitance and ESR required for a specified

ripple using the following equations:

[ ]

OUT

LPEAK

OPTIMUM

OUT

S

OUT

OUT

IN_MIN

OUT

OUT

I

I

x L

C

[ F]

t

0.5 x V

(V

V

)

0.5 x V

ESR m

I





µ =

−





∆

−









∆

Ω =

For very-low-output-ripple applications, the output of the

boost converter can be followed by an RC filter to further

reduce the ripple. Figure 2 shows a 100I, 0.1FF (RF CF)

output voltage regulation resistive divider must remain

connected to the diode/output capacitor node.

Use X7R ceramic capacitors for more stability over the

full temperature range.

Input-Capacitor Selection

Bypass IN to PGND with a 1FF (min) ceramic capacitor.

Depending on the supply source impedance, higher val-

ues may be needed. Make sure that the input capacitors

are close enough to the IC to provide adequate decou-

pling at IN as well. If the layout cannot achieve this,

add another 0.1FF ceramic capacitor between IN and

PGND in the immediate vicinity of the IC. Bulk aluminum

electrolytic capacitors may be needed to avoid chatter-

ing at low-input voltage. In case of aluminum electrolytic

capacitors, calculate the capacitor value and ESR of the

input capacitor using the following equations:

[ ]

OUT

OUT

LPEAK

OPTIMUM

OUT

IN

S

IN_MIN

IN

IN_MIN OUT

IN_MIN

IN

IN_MIN

OUT

OUT

V

x I

I

x L

x V

C [ F]

t

x V

x 0.5 x V

V

(V

V

)

0.5 x V

x x V

ESR m

V

x I





µ =

−





η

∆

−









∆

η

Ω =

Figure 2. Typical Operating Circuit with RC Filter

CNTRL

SHDN

IN

SGND

TO 5.5V

PGND

L1

FB

BIAS

LX

D1

MAX15059