MAX1653EEE datasheet(17/28 Pages) MAXIM

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High-Efficiency, PWM, Step-Down

DC-DC Controllers in 16-Pin QSOP

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flow, and thereby suppresses discontinuous-mode

inductor ringing by changing the reverse current-limit

detection threshold from 0 to -100mV, allowing the

inductor current to reverse at very light loads.

In most applications, SKIP should be tied to GND in

order to minimize quiescent supply current. Supply cur-

rent with SKIP high is typically 10mA to 20mA, depend-

ing on external MOSFET gate capacitance and

switching losses.

Forced continuous conduction via SKIP can improve

cross regulation of transformer-coupled multiple-output

supplies. This second function of the SKIP pin produces

a result that is similar to the method of adding sec-

ondary regulation via the SECFB feedback pin, but with

much higher quiescent supply current. Still, improving

cross regulation by enabling SKIP instead of building in

SECFB feedback can be useful in noise-sensitive appli-

cations, since SECFB and SKIP are mutually exclusive

pins/functions in the MAX1652 family.

Adjustable-Output Feedback

(Dual-Mode FB Pin)

The MAX1652–MAX1655 family has both fixed and

adjustable output voltage modes. For fixed mode, con-

nect FB to GND for a 3.3V output and to VL for a 5V out-

put. Adjusting the main output voltage with external

resistors is easy for any of the devices in this family, via

the circuit of Figure 6. The feedback voltage is nominal-

ly 2.5 for all family members except the MAX1655,

which has a nominal FB voltage of 1V. The output volt-

age (given by the formula in Figure 6) should be set

approximately 2% high in order to make up for the

MAX1652’s load-regulation error. For example, if

designing for a 3.0V output, use a resistor ratio that

results in a nominal output voltage of 3.06V. This slight

offsetting gives the best possible accuracy.

Recommended normal values for R5 range from 5k

Ω to

100k

Ω.

Remote sensing of the output voltage, while not possi-

ble in fixed-output mode due to the combined nature of

the voltage- and current-sense input (CSL), is easy to

achieve in adjustable mode by using the top of the

external resistor divider as the remote sense point.

Duty-Factor Limitations for

Low VOUT/VIN Ratios

The MAX1652/MAX1653/MAX1654’s output voltage is

adjustable down to 2.5V and the MAX1655’s output is

adjustable as low as 1V. However, the minimum duty

factor may limit the choice of operating frequency, high

input voltage, and low output voltage.

MAX1652

MAX1653

MAX1654

MAX1655

CSL

CSH

GND

MAIN

OUTPUT

REMOTE

SENSE

LINES

VOUT

WHERE VREF (NOMINAL) = 2.5V (MAX1652–MAX1654)

= 1.0V (MAX1655)

= VREF (1 + –––)

V+

Figure 6. Adjusting the Main Output Voltage

SHDN

SKIP

LOAD

CURRENT

MODE

NAME

DESCRIPTION

Low

Shutdown

All circuit blocks

turned off; supply

current = 3µA typ

High

Low

Low,

<10%

Idle

Pulse-skipping;

supply current =

300µA typ at VIN =

10V; discontinuous

inductor current

High

Low

Medium,

<30%

Idle

Pulse-skipping;

continuous inductor

current

High

Low

High,

>30%

PWM

Constant-frequency

PWM; continuous

inductor current

High

Low Noise*

(PWM)

Constant-frequency

PWM regardless of

load; continuous

inductor current

even at no load

Table 3. Operating-Mode Truth Table

MAX1652/MAX1654 have no SKIP pin and therefore can’t go

into low-noise mode.

X = Don’t care

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