MAX1896 Просмотр технического описания (PDF) - Maxim Integrated
Номер в каталоге
Компоненты Описание
производитель
MAX1896 Datasheet PDF : 10 Pages
| |||
1.4MHz SOT23 Current-Mode
Step-Up DC-DC Converter
The inductance (H) value is then given by:
L
=
[VIN(MIN)2 x η x (VOUT − VIN(MIN))]
VOUT2 x LIR x IOUT(MAX) x fOSC
Diode Selection
The output diode should be rated to handle the output
voltage and the peak switch current. Make sure the
diode’s peak current rating is at least IPK and that its
breakdown voltage exceeds VOUT. Schottky diodes are
recommended. If a junction rectifier is used, it must be
an ultra-fast type (trr < 50ns) to prevent excessive loss
in the rectifier.
Input and Output Capacitor Selection
The MAX1896 operates with both tantalum and ceramic
output capacitors. When using tantalum capacitors, the
zero caused by the ESR of the tantalum is used to
ensure stability. When using ceramic capacitors, the
zero due to the ESR will be at too high a frequency to
be useful in stabilizing the control loop. When using
ceramic capacitors, use a feedforward capacitor to
increase the phase margin, improving the control-loop
stability. Figure 3 shows the circuit with ceramic capac-
itors and the feedforward capacitor, CFF. Use the fol-
lowing equation to determine the value of the
feedforward capacitor:
CFF
=
k1
x
COUT
x
VOUT2
0.5
R1
VIN
where:
k1
=
7.14
x 10−4 with units of
Ω x
A
F 0.5
R1 = see Figure 3, (Ω)
COUT = total output capacitance including any bypass
capacitor on the output bus, (Farads). See Figure 3.
VOUT = output voltage, (V)
VIN = input voltage, (V).
Setting the Output Voltage
The MAX1896 operates with an adjustable output from
VIN to 13V. Connect a resistive voltage-divider from the
output to FB (see Typical Operating Circuit). Choose a
value for R2 between 10kΩ and 50kΩ. Calculate R1
using the equation:
R1 =
R2
x
VOUT
VFB
− 1
where VFB, the step-up regulator feedback set point, is
1.24V. Connect the resistive-divider as close to the IC
as possible.
Soft-Start Capacitor
The soft-start capacitor should be large enough that the
current limit does not reach final value before the output
has reached regulation. Calculate CSS to be:
CSS > k2 x COUT x
VIN
VOUT2 −
x IINRUSH
VIN x VOUT
− IOUT x VOUT
where:
k2 = 21 x 10-6, (S)
VOUT = maximum output voltage, (V)
IINRUSH = peak inrush current allowed, (A)
IOUT = maximum output current during power-up stage, (A)
VIN = minimum input voltage, (V)
The soft-start duration (tSS) is the time it takes the cur-
rent limit to reach its final value. The soft-start duration
can be calculated by the equation:
tss = k3 ✕ CSS
where:
k3 = 6.67 ✕ 105Ω
_______________________________________________________________________________________ 7
Share Link: 