Printed Circuit Layouts

A good layout is essential for clean, stable operation.

The layouts and component placement diagrams given

in Figures 4, 5, 6, and 7 have been successfully tested

over a wide range of operating conditions. Note that the

1µF bypass capacitor (C2) must be positioned as close

to the V+ and GND pins as possible. Also, place the out-

put capacitor as close to the OUT and GND pins as pos-

sible. The traces connecting the input and output filter

capacitors and the catch diode must be short to mini-

mize inductance and capacitance. For this reason, avoid

using sockets, and solder the IC directly to the PC

board. Use an uninterrupted ground plane if possible.

Output-Ripple Filtering

A simple lowpass pi-filter (Figure 3) can be added to

the output to reduce output ripple to about 5mVp-p.

The cutoff frequency shown is 21kHz. Since the filter

inductor is in series with the circuit output, its resis-

tance should be minimized so the voltage drop across

it is not excessive.

5V, Step-Down,

Current-Mode PWM DC-DC Converters

______________________________________________________________________________________

11

Table 2. Component Values and Suppliers

MAX730AC/MAX738AC/MAX744AC

Commercial Temp. Range

MAX730AE/M, MAX738AE/M, MAX744AE/M

Wide Temp. Range

Surface

Mount

C3 = 68µF, 16V

C4 = 100µF, 6.3V

Matsuo (714) 969-2491

267 series

Sprague (603) 224-1961

595D/293D series

L1 = 33µH

Sumida (708) 956-0666

CD54-330N (MAX730AC)

CD105-330N

(MAX738AE/M, MAX744AE/M)

Coiltronics (407) 241-7876

CTX50 series

C3 = 68µF, 16V

C4 = 100µF, 6.3V

Matsuo (714) 969-2491

267 series

Sprague (603) 224-1961

595D/293D series

Miniature

Through-

Hole

L1 = 33µH to 100µH

Sumida (708) 956-0666

RCH654-101K (MAX730A)

RCH895-101K

(MAX738A/MAX744A)

C3 = 150µF, 16V

C4 = 150µF, 16V or

390µF, 6.3V

Nichicon (708) 843-7500

PL series

Low-ESR electrolytics

L1 = 33µH

Sumida (708) 956-0666

RCH654-330M (MAX730A)

RCH895-330M

(MAX738A/MAX744A)

Low-Cost

Through-

Hole

Inductors

Capacitors

Inductors

Capacitors

Production

Method

L1 = 33µH to 100µH

Sumida (708) 956-0666

CD54-101KC (MAX730AC)

CD105-101KC

(MAX738AC/MAX744AC)

Coiltronics (407) 241-7876

CTX100 series

C3 = 150µF, 16V

C4 = 220µF, 10V

Sanyo (619) 661-6322

OS-CON series

Low-ESR

organic semiconductor

(Rated from -55°C to +105°C)

Mallory (317) 273-0090

THF series

C3 = 100µF, 20V

C4 = 220µF, 10V

(Rated from -55°C to +125°C)

L1 = 100µH

Maxim

MAXL001

100µH iron-power toroid

Renco (516) 586-5566

RL1284-100

C3 = 150µF, 16V

C4 = 390µF, 6.3V

Maxim

MAXC001

150µF, low-ESR

electrolytic

United Chemicon

(708) 843-7500