MAX1932ETC+T Maxim Integrated Products, MAX1932ETC+T Datasheet - Page 7

• Manufacturer Part Number: MAX1932ETC+T
• Description: IC SUPPLY BIAS APD 12-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX1932ETC+T

• Applications: Controller, Fiber Optic Network
• Voltage - Input: 2.7 ~ 5.5 V
• Number Of Outputs: 1
• Voltage - Output: 4.5 ~ 90 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 12-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Many biasing applications require an adjustable output
voltage, which is easily obtained using the MAX1932’s
DAC output (Figure 2).
The DAC output voltage is given by the following equation:
On power-up, DACOUT defaults to FF hex or 1.25V,
which corresponds to the minimum V
The voltage generated at DACOUT is coupled to FB
through R6. DACOUT can sink only 50µA so:
Select the minimum output voltage (V
maximum output voltage (V
adjustment range. R5 sets the adjustment span using
the following equation:
R8 sets the minimum output of the adjustment range
with the following equation:
Adjust the output voltage by connecting a voltage-
divider from the output (V
omitted). Select R8 between 10kΩ to 50kΩ. Calculate
R5 with the following equation:
Optimum inductor selection depends on input voltage,
output voltage, maximum output current, switching fre-
quency, and inductor size. Inductors are typically spec-
ified by their inductance (L), peak current (I
resistance (L
Output and DAC Adjustments Range
V
DACOUT
R5 = (V
Setting the Output Voltage without
R
R8 = (1.25V
).
R
OUTFF
5
_______________________________________________________________________________________
=
=
CODE
R
R
8
6
⎛
⎜
⎝
- V
≥
VOUT
1 25
OUT
1 25
✕
.
50
×
OUT01
.
R5)/(V
⎛
⎜
⎝
Digitally Controlled, 0.5% Accurate,
μ
1 25
V
) to FB (Figure 2 with R6
A
OUT01
256
V
.
Inductor Selection
−
) (R6/1.25V)
1
V
⎞
⎟
⎠
OUTFF
⎞
⎟ +
⎠
OUT
) for the desired
⎛
⎜
⎝
OUTFF
1 25
output voltage.
)
256
.
V
the DAC
), and the
⎞
⎟
⎠
PK
), and
Safest APD Bias Supply
The inductance value is given by:
where V
mum output current delivered, V
age, and T is the switching period (3.3µs), η is the
estimated power conversion efficiency, and D is the
maximum duty cycle:
Since the L equation factors in efficiency, for inductor cal-
culation purposes, an η of 0.5 to 0.75 is usually suitable.
For example, with a maximum DC load current of 2.5mA,
a 90V output, V
mated at 0.75, the above equation yields an L of 111µH,
so 100µH would be a suitable value.
The peak inductor current is given by:
These are typical calculations. For worst case, refer to
the article titled “Choosing the MAX1932 External
Indicator, Diode, Current Sense Resistor, and Output
Filter Capacitor for Worst Case Conditions” located on
the Maxim website in the Application Notes section (visit
www.maxim-ic.com/an1805).
An N-FET power switch is required for the MAX1932. The
N-FET switch should be selected to have adequate on-
resistance with the MOSFET V
down voltage of the N-FET must be greater than V
For higher-current output applications (such as 5mA at
90V), SOT23 high-voltage low-gate-threshold N-FETs
may not have adequate current capability. For example,
with a 5V input, a 90V, 5mA output requires an inductor
peak of 240mA. For such cases it may be necessary to
simply parallel two N-FETs to achieve the required cur-
rent rating. With SOT23 devices this often results in
smaller and lower cost than using a larger N-FET device.
The output diode should be rated to handle the output
voltage and the peak switch current. Make sure that the
diode’s peak current rating is at least I
breakdown voltage exceeds V
ery time (t
D < (V
IN
External Power-Transistor Selection
OUT
is the input voltage, I
rr
< 10ns) and low junction capacitance
- V
IN
L
=
IN
= 5V, D = 0.9, T = 3.3µs, and η esti-
I
2
PK
)/V
( )
I
V
OUT MAX
IN
OUT
=
V
2
(
IN
×
up to a maximum of 0.9
D
× ×
L
2
D T
GS
)
OUT
×
× ×
V
OUT
T
OUT(MAX)
= V
OUT
. Fast reverse recov-
Diode Selection
η
IN(MIN)
is the output volt-
PK
. The break-
is the maxi-
and that its
OUT
.
7