Rainbow Electronics MAX8614B - Manual (Page 11)

MAX8614A/MAX8614B
Dual-Output (+ and -) DC-DC
Converters for CCD
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the inverter output, load disconnect is implemented by
turning off the inverter’s internal power switch.
Current-Limit Select
The MAX8614B allows an inductor current limit of 0.8A
on the step-up converter and 0.75A on the inverter. The
MAX8614A allows an inductor current limit of 0.44A on
the step-up converter and 0.33A on the inverter. This
allows flexibility in designing for higher load-current
applications or for smaller, more compact designs
when less power is needed. Note that the currents list-
ed above are peak inductor currents and not output
currents. The MAX8614B output current is 50mA at
+15V and 100mA at -7.5V. The MAX8614A output cur-
rent is 25mA at +15V and 50mA at -7.5V.
Load Transient/Voltage Positioning
The MAX8614A/MAX8614B match the load regulation to
the voltage droop seen during load transients. This is
sometimes called voltage positioning. This results in min-
imal overshoot when a load is removed and minimal volt-
age drop during a transition from light load to full load.
The use of voltage positioning allows superior load-
transient response by minimizing the amplitude of over-
shoot and undershoot in response to load transients.
DC-DC converters with high control-loop gains maintain
tight DC load regulation but still allow large voltage
drops of 5% or greater for several hundred microsec-
onds during transients. Load-transient variations are
seen only with an oscilloscope (see the Typical
Operating Characteristics). Since DC load regulation is
read with a voltmeter, it does not show how the power
supply reacts to load transients.
Applications Information
Adjustable Output Voltage
The positive output voltage is set by connecting FBP to
a resistive voltage-divider between the output and GND
(Figure 1). Select feedback resistor R2 in the 30k
Ω to
100k Ω range. R1 is then given by:
where V FBP = 1.01V.
The negative output voltage is set by connecting FBN
to a resistive voltage-divider between the output and
REF (Figure 1). Select feedback resistor R4 in the 30k
Ω
to 100k
Ω range. R3 is then given by:
where V REF = 1.25V and V FBN = 0V.
Inductor Selection
The MAX8614A/MAX8614B high switching frequency
allows for the use of a small inductor. The 4.7µH and
2.2µH inductors shown in the Typical Operating Circuit is
recommended for most applications. Larger inductances
reduce the peak inductor current, but may result in skip-
ping pulses at light loads. Smaller inductances require
less board space, but may cause greater peak current
due to current-sense comparator propagation delay.
Use inductors with a ferrite core or equivalent. Powder
iron cores are not recommended for use with high switch-
ing frequencies. The inductor’s incremental saturation rat-
ing must exceed the selected current limit. For highest
efficiency, use inductors with a low DC resistance (under
200m
Ω); however, for smallest circuit size, higher resis-
tance is acceptable. See Table 1 for a representative list
of inductors and Table 2 for component suppliers.
Diode Selection
The MAX8614A/MAX8614B high switching frequency
demands a high-speed rectifier. Schottky diodes, such
as the CMHSH5-2L or MBR0530L, are recommended.
Make sure that the diode’s peak current rating exceeds
the selected current limit, and that its breakdown volt-
age exceeds the output voltage. Schottky diodes are
preferred due to their low forward voltage. However,
ultrahigh-speed silicon rectifiers are also acceptable.
Table 2 lists component suppliers.
Capacitor Selection
Output Filter Capacitor
The primary criterion for selecting the output filter
capacitor is low effective series resistance (ESR). The
product of the peak inductor current and the output fil-
ter capacitor’s ESR determines the amplitude of the
high-frequency ripple seen on the output voltage.
These requirements can be balanced by appropriate
selection of the current limit.
For stability, the positive output filter capacitor, C1,
should satisfy the following:
C1 > (6L I
BSTMAX
) / ( R
CS
D+ V
BST
2 )
where R CS = 0.015 (MAX8614B), and 0.035 (MAX8614A).
D+ is 1 minus the step-up switch duty cycle and is:
D+ = V CC / V BST
R
V V
V V
FBN IMV
REF FBN
3 4
= ×
−
−
⎛
⎝⎜ ⎞
⎠⎟
R
R
V
V
BST
FBP
1 2 1
= −
⎛
⎝⎜ ⎞
⎠⎟
R