MAX15059 Datasheet, PDF (12/15 Pages) Maxim Integrated Products – 76V, 300mW Boost Converter and Current Monitor for APD Bias Applications

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:
R₂
=

R_{1}

V_OUT
V_REF




− 1_

where V_OUT can range from (V_IN + 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:
I_LPEAK =
2 × t _S × (V_OUT − V_{IN _MIN}) ×I_{OUT_MAX}
η×L
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:
L
MIN^[µH]
=
2
×
t
S
×
I_OUT × (V_OUT
η ×I_{L2IM_LX}
−
V_{IN_MIN})
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:
where t_S is the switching period in Fs, V_OUT 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).
MAX15059
FB
VOUT
R2
VCNTRL > 1.3V, VFB = 1.23V
VCNTRL < 1.2V, VFB = VCNTRL
R1
L
OPTIMUM^[µH]
=
L
MAX ^[µH]
2.25
where:
L _MAX [µH]
=
V_{I2N_MIN}(V_OUT −
2 ×I_OUT
V_{IN_MIN})
× V_O2UT
×
t_S
×
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 V_IN = 2.8V, V_OUT
= 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
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