MAX1627CSLASHD datasheet(12/16 Pages) MAXIM

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MAX1626/MAX1627

5V/3.3V or Adjustable,

100% Duty-Cycle, High-Efficiency,

Step-Down DC-DC Controllers

Maxim Integrated | 12

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Place a surface-mount ceramic capacitor very close to

V+ and GND, as shown in Figure 7. This capacitor

bypasses the MAX1626/MAX1627, and prevents spikes

and ringing on the power source from obscuring the

current feedback signal and causing jitter. 0.47µF is

recommended. Increase the value as necessary in

high-power applications.

Bypass REF with 0.1µF. This capacitor should be

placed within 0.2 inches (5mm) of the IC, next to REF,

with a direct trace to GND (Figure 7).

Layout Considerations

High-frequency switching regulators are sensitive to PC

board layout. Poor layout introduces switching noise into

the current and voltage feedback signals, resulting in jit-

ter, instability, or degraded performance. The current-

sense resistor must be placed within 0.2 inches (5mm)

of the controller IC, directly between V+ and CS. Place

voltage feedback resistors (MAX1627) next to the FB pin

(no more than 0.2") rather than near the output. Place

the 0.47µF input and 0.1µF reference bypass capacitors

within 0.2 inches (5mm) of V+ and REF, and route

directly to GND. Figure 7 shows the recommended lay-

out and routing for these components.

High-power traces, highlighted in the

Typical Operating

Circuit (Figure 1), should be as short and as wide as

possible. The supply-current loop (formed by C2, C3,

RSENSE, U1, L1, and C1) and commutation-current loop

(D1, L1, and C1) should be as tight as possible to

reduce radiated noise. Place the anode of the commuta-

tion diode (D1) and the ground pins of the input and

output filter capacitors close together, and route them to

a common “star-ground” point. Place components and

route ground paths so as to prevent high currents from

causing large voltage gradients between the ground pin

of the output filter capacitor, the controller IC, and the

reference bypass capacitor. Keep the extra copper on

the component and solder sides of the PC board, rather

than etching it away, and connect it to ground for use as

a pseudo-ground plane. Refer to the MAX1626

Evaluation Kit manual for a two-layer PC board example.

Stability and MAX1627 Feedback

Compensation

Use proper PC board layout and recommended exter-

nal components to ensure stable operation. In one-

shot, sequenced PFM DC-DC converters, instability is

manifested as “Motorboat Instability.” It is usually

caused by excessive noise on the current or voltage

feedback signals, ground, or reference, due to poor PC

board design or external component selection.

Motorboat instability is characterized by grouped

switching pulses with large gaps and excessive low-

frequency output ripple. It is normal to see some

grouped switching pulses during the transition from

discontinuous to continuous current mode. This effect

is associated with small gaps between pulse groups

and output ripple similar to or less than that seen dur-

ing no-load conditions.

Instability can also be caused by excessive stray capaci-

tance on FB when using the MAX1627. Compensate for

this by adding a 0pF to 330pF feed-forward capacitor

across the upper feedback resistor (R2 in Figure 5).

MAX1626/MAX1627 vs.

MAX1649/MAX1651 vs.

MAX649/MAX651

The MAX1626/MAX1627 are specialized, third-genera-

tion upgrades to the MAX649/MAX651 step-down con-

trollers. They feature improved efficiency, a reduced

current-sense threshold (100mV), soft-start, and a 100%

duty cycle for lowest dropout. The MAX649/ MAX651

have a two-step (210mV/110mV) current-sense thresh-

old. The MAX1649/MAX1651 are second-generation

upgrades with a 96.5% maximum duty cycle for

improved dropout performance and a reduced current-

sense threshold (110mV) for higher efficiency, especially

at low input voltages. The MAX1649/ MAX1651 are

preferable for special applications where a 100% duty

cycle is undesirable, such as flyback and SEPIC circuits.

Since the MAX1626’s pinout is similar to those of the

MAX649 and MAX1649 family parts, the MAX1626 can

be substituted (with minor external component value

changes) into fixed-output mode applications, provided

the PC board layout is adequate. The MAX1627 can

also be substituted when MAX649 or MAX1649 family

parts are used in adjustable mode, but the feedback

resistor values must be changed, since the MAX1627

has a lower reference voltage (1.3V vs. 1.5V). Reduce

MAX1626

C REF

C V+ BYPASS

SCALE

RSENSE

Figure 7. Recommended Placement and Routing of the

Current-Sense Resistor, 0.1µF Reference, and 0.47µF Input

Bypass Capacitors

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