MAX15031 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX15031

80V, 300mW Boost Converter and Current Monitor for APD Bias Applications

Maxim Integrated 

80V, 300mW Boost Converter and Current

Monitor for APD Bias Applications

FB

MAX15031

VOUT

R2

R1

Figure 1. Adjustable Output Voltage

Design Procedure

Setting the Output Voltage

Set the MAX15031 output voltage by connecting a resis-

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

Select R1 (FB to SGND resistor) between 200kΩ and

400kΩ. Calculate R2 (VOUT to FB resistor) using the fol-

lowing equation:

R2

=

⎡⎛

R1 ⎣⎢⎢⎝⎜

VOUT

VREF

⎞

⎠⎟

⎤

− 1⎥

⎦⎥

where VOUT can range from (VIN + 1V) to 76V and VREF

= 1.245V or VCNTRL depending on the VCNTRL value.

For VCNTRL > 1.5V, the internal 1.245V (typ) reference

voltage is used as the feedback set point (VREF =

1.245V) and for VCNTRL < 1.25V, VREF = VCNTRL.

Determining Peak Inductor Current

If the boost converter remains in the discontinuous

mode of operation, then the approximate peak inductor

current, ILPEAK (in amperes), is represented by the for-

mula below:

ILPEAK =

2 × TS × (VOUT − VIN_MIN) × IOUT_MAX

η×L

where TS is the switching period in microseconds,

VOUT is the output voltage in volts, VIN_MIN is the mini-

mum input voltage in volts, IOUT_MAX is the maximum

output current in amperes, L is the inductor value in

microhenrys, and η is the efficiency of the boost con-

verter (see the Typical Operating Characteristics).

Determining the Inductor Value

Three key inductor parameters must be specified for

operation with the MAX15031: 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 switch peak

current-limit value (ILIM_LX = 1.6A). Choose an inductor

with a low-DCR resistance 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 discontinuous mode operation for supplying full

300mW of output power.

LMIN[μH]

=

2

×

TS

× IOUT × (VOUT

η × IL2IM_LX

−

VIN_MIN)

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

amperes) are typical values (so that efficiency is opti-

mum for typical conditions), TS (in microseconds) is the

period, η is the efficiency, and ILIM_LX is the peak

switch current in amperes (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 DCM

using the following formula:

LOPTIMUM[μH]

=

LMAX [μH]

2.25

where

LMAX [μH]

=

VI2N_MIN(VOUT − VIN_MIN) ×

2 × IOUT × VO2UT

TS

×

η

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

3mA, η = 45%, ILIM_LX = 1.3A, and TS = 2.5μs: LMIN =

1.3μH and LMAX = 23μH.

For a worse-case scenario in which VIN = 2.9V, VOUT =

70V, IOUT= 4mA, η = 43%, ILIM_LX= 1.3A, and TS =

2.5μs: LMIN = 1.8μH and LMAX = 15μH.

The choice of 4.7μH 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.

12 ______________________________________________________________________________________

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