MAX15059 datasheet(12/15 Pages) MAXIM

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12 _____________________________________________________________________________________

76V, 300mW Boost Converter and Current

Monitor for APD Bias Applications

Design Procedure

Setting the Output Voltage

Set the MAX15059 output voltage by connecting a resis-

tive divider from the output to FB to SGND (Figure 1).

Select R1 (FB to SGND resistor) between 5kI and 10kI.

Calculate R2 (VOUT to FB resistor) using the following

equation:

OUT

REF









−





















where VOUT can range from (VIN + 5V) to 76V. Apply a

voltage to the CNTRL input to set the feedback set-point

reference voltage, VREF (see the Functional Diagram).

For VCNTRL > 1.3V, the internal 1.23 (typ) reference volt-

age is used as the feedback set point and for VCNTRL <

1.2V, VREF = VCNTRL. See the Adjusting the Feedback

Set-Point/Reference Voltage section for more information

on adjusting the feedback reference voltage, VREF.

Determining Peak Inductor Current

If the boost converter remains in the discontinuous mode

of operation, then the approximate peak inductor cur-

rent, ILPEAK (in A), is represented by the formula below:

OUT

IN_MIN

OUT_MAX

LPEAK

2 t

) I

−

η×

where tS is the switching period in Fs, VOUT is the output

voltage in volts, VIN_MIN is the minimum input voltage

in volts, IOUT_MAX is the maximum output current in

amps, L is the inductor value in FH, and E is the effi-

ciency of the boost converter (see the Typical Operating

Characteristics).

Determining the Inductor Value

Three key inductor parameters must be specified for

operation with the MAX15059: inductance value (L),

inductor saturation current (ISAT), and DC resistance

(DCR). In general, the inductor should have a saturation

current rating greater than the maximum peak switch

current-limit value (ILIM_LX = 1.3A). DC series resistance

(DCR) should be be low for reasonable efficiency.

Use the following formula to calculate the lower bound of

the inductor value at different output voltages and output

currents. This is the minimum inductance value for dis-

continuous mode operation for supplying full 300mW of

output power:

OUT

IN_MIN

MIN

LIM_LX

2 t

)

[ H]

−

η×

where VIN_MIN, VOUT (both in volts), and IOUT (in amps)

are typical values (so that efficiency is optimum for typi-

cal conditions), tS (in Fs) is the period, E is the efficiency,

and ILIM_LX is the peak switch current in amps (see the

Electrical Characteristics table).

Calculate the optimum value of L (LOPTIMUM) to ensure

the full output power without reaching the boundary

between continuous-conduction mode (CCM) and dis-

continuous-conduction mode (DCM) using the following

formula:

MAX

OPTIMUM

[ H]

2.25

where:

OUT

IN_MIN

MAX

OUT

) t

[ H]

2 I

−

× η

For a design in which VIN = 3.3V, VOUT = 70V, IOUT =

3mA, E = 45%, ILIM_LX = 1.2A, and tS = 2.5Fs: LMAX =

27FH and LMIN = 1.5FH.

For a worse-case scenario in which VIN = 2.8V, VOUT

= 70V, IOUT = 4mA, η = 43%, ILIM_LX = 1.2A, and tS =

2.5Fs: LMAX = 15FH and LMIN = 2.2FH.

The choice of 4.7FH is reasonable given the worst-case

scenario above. In general, the higher the inductance,

the lower the switching noise. Load regulation is also

better with higher inductance.

Figure 1. Adjustable Output Voltage

VCNTRL > 1.3V, VFB = 1.23V

VCNTRL < 1.2V, VFB = VCNTRL

MAX15059

VOUT

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