Low-Noise, 14V Input, 1A, PWM

Step-Down Converters

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11

The output current capability for each mode is deter-

mined by the following equations:

modes)

where:

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 (see the 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 startup, when

nects CVL to BOOT. This bootstrap mechanism causes

the internal circuitry to be supplied from the output and

thereby reduces the input quiescent current by a factor

put voltage exceeds 5.5V. Instead, connect BOOT to

AGND to keep CVL regulated at 3V.

CVL has a 5mA capability to supply external logic cir-

cuitry and is disabled in shutdown mode.

Applications Information

Output Voltage Selection

Connect FB to AGND to select the internal 3.3V output

mode. Connect BOOT to OUT in this configuration.

connect FB to a resistor voltage-divider between the

output and AGND (Figure 3). Select R2 in the 20k

Ω to

100k

Ω range. Calculate R1 as follows:

Connect a small capacitor across R1 to compensate for

stray capacitance at the FB pin:

where: R2 = 100k

Ω, use 4.7pF.

Inductor Selection

The MAX1684/MAX1685s’ high switching frequency

allows the use of small surface-mount inductors. Table 2

shows a selection of suitable inductors for different out-

put voltage ranges. Calculate the minimum inductor by:

where:

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/MAX1685s’ 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 capac-

itors, as they typically have high ESR. Low-ESR alu-

minum 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-current ratings of tantalum capacitors. Ceramic

capacitors offer the lowest ESR overall. Sanyo OS-CON

C1

5 (10

R

7

2

=

−

)

MAX1684

MAX1685

FB

R1

C1

R2

Figure 3. Setting Output Voltage