Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
The output current capability for each mode is deter-
mined by the following equations:
I
OUTMAX
= I
LIM
- 0.5
·
I
RIPPLE
(for PWM and normal
modes)
I
OUTMAX
= 0.5
·
I
LIMLP
(for low-power mode)
where:
I
RIPPLE
= ripple current =
(V
IN
- V
OUT
)
·
V
OUT
/ (V
IN
·
f
OSC
·
L)
I
LIM
= current limit in PWM mode
I
LIMLP
= current limit in low-power mode.
MAX1684/MAX1685
V
OUT
MAX1684
MAX1685
FB
R1
C1
R2
Internal Low-Voltage Regulators and
Bootstrap (BOOT)
The MAX1684/MAX1685 have two internal regulators
(VH and VL) that generate low-voltage supplies for
internal circuitry (Functional
Diagram).
The VH regulator
generates -4.6V with respect to IN to supply the P-
channel switch and driver. Bypass CVH to IN with a
0.1µF capacitor. The VL regulator generates a 3V out-
put at CVL to supply internal low-voltage blocks, as well
as the N-channel switch and driver. Bypass CVL to
AGND with a 1µF capacitor.
To reduce the quiescent current in low-power and nor-
mal modes, connect BOOT to OUT. After start-up,
when V
BOOT
exceeds 2.6V, the internal bootstrap
switch connects CVL to BOOT. This bootstrap mecha-
nism causes the internal circuitry to be supplied from
the output and thereby reduces the input quiescent
current by a factor of V
OUT
/ V
IN
. Do not connect BOOT
to OUT if the output voltage exceeds 5.5V. Instead,
connect BOOT to AGND to keep CVL regulated at 3V.
Apply an external supply voltage, such as +3.3V or +5V,
to BOOT to achieve a bootstrap effect when the output
voltage is too low to bootstrap the device (below 2.7V).
CVL has a 5mA capability to supply external logic cir-
cuitry and is disabled in shutdown mode.
Figure 3. Setting Output Voltage
Connect a small capacitor across R1 to compensate for
stray capacitance at the FB pin.
C1
=
5 (10
−
7
)
R
2
For R2 = 100kΩ, use 4.7pF.
Inductor Selection
The MAX1684/MAX1685’s high switching frequency
allows the use of small surface-mount inductors. Table
2 shows a selection of suitable inductors for different
output voltage ranges. Calculate the minimum inductor
by:
L = 0.9(V
OUT
- 0.3V) / (I
RIPPLE MAX
·
f
OSC
)
where:
I
RIPPLE MAX
= should be less than or equal to 400mA
f
OSC
= 300kHz (MAX1684) or 600kHz (MAX1685)
Capacitor Selection
Select input and output filter capacitors to service
inductor currents while minimizing voltage ripple. The
input filter capacitor reduces peak currents and noise at
the voltage source. The MAX1684/MAX1685’s loop gain
is relatively low, to enable the use of small, low-value
output filter capacitors. Higher capacitor values provide
improved output ripple and transient response.
Low-ESR capacitors are recommended. Capacitor ESR
is a major contributor to output ripple (usually more than
60%). Avoid ordinary aluminum electrolytic capacitors,
as they typically have high ESR. Low-ESR aluminum
electrolytic capacitors are acceptable and relatively
inexpensive. Low-ESR tantalum capacitors are better
and provide a compact solution for space-constrained
surface-mount designs. Do not exceed the ripple-cur-
rent ratings of tantalum capacitors. Ceramic capacitors
11
________________Applications Information
Output Voltage Selection
Connect FB to AGND to select the internal 3.3V output
mode. Connect BOOT to OUT in this configuration.
To select an output voltage between 1.25V and V
IN
,
connect FB to a resistor voltage divider between the
output and AGND (Figure 3). Select R2 in the 20kΩ to
100kΩ range. Calculate R1 as follows:
R1 = R2 [( V
OUT
/ V
FB
) - 1]
where V
FB
= 1.25V.
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