MAX797CSLASHD datasheet(17/32 Pages) MAXIM

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Step-Down Controllers with

Synchronous Rectifier for CPU Power

______________________________________________________________________________________

quency where harmonics of the switching frequency

don’t overlap a sensitive frequency band. If necessary,

synchronize the oscillator to a tight-tolerance external

clock generator.

The low-noise mode (SKIP = high) forces two changes

upon the PWM controller. First, it ensures fixed-frequen-

cy operation by disabling the minimum-current com-

parator and ensuring that the PWM latch is set at the

beginning of each cycle, even if the output is in regula-

tion. Second, it ensures continuous inductor current

flow, and thereby suppresses discontinuous-mode

inductor ringing by changing the reverse current-limit

detection threshold from zero 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 pro-

duces a result that is similar to the method of adding

secondary 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 applications, since SECFB and SKIP are

mutually exclusive pins/functions in the MAX796 family.

Adjustable-Output Feedback

(Dual-Mode FB Pin)

Adjusting the main output voltage with external resis-

tors is easy for any of the devices in the MAX796 family,

via the circuit of Figure 6. The nominal output voltage

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

imately 2% high in order to make up for the MAX796’s

-2.5% typical 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. To achieve a 2.505V nominal output, simply

connect FB to CSL directly. To achieve output voltages

lower than 2.5V, use an external reference-voltage

source higher than VREF, as shown in Figure 7. For best

accuracy, this second reference voltage should be

much higher than VREF. Alternatively, an external op

amp could be used to gain-up REF in order to create

the second reference source. This scheme requires a

minimum load on the output in order to sink the R3/R4

divider current.

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.

Fixed-output accuracy is guaranteed to be ±4% over

all conditions. In special circumstances, it may be nec-

essary to improve upon this output accuracy. The High-

Accuracy Adjustable-Output Application (Figure 18)

provides ±2.5% accuracy by adding an integrator-type

error amplifier.

The breakdown voltage rating of the current-sense

inputs (7V absolute maximum) determines the 6V maxi-

mum output adjustment range. To extend this output

range, add two matched resistor dividers and speed-

up capacitors to form a level translator, as shown in

Figure 8. Be sure to set these resistor ratios accurately

(using 0.1% resistors), to avoid adding excessive error

to the 100mV current-limit threshold.

Secondary Feedback-Regulation Loop

(SECFB Pin)

A flyback winding control loop regulates a secondary

winding output (MAX796/MAX799 only), improving

cross-regulation when the primary is lightly loaded or

when there is a low input-output differential voltage. If

SECFB crosses its regulation threshold (VREF for the

MAX796

MAX797

MAX799

CSL

CSH

GND

MAIN

OUTPUT

REMOTE

SENSE

LINES

VOUT

WHERE VREF (NOMINAL) = 2.505V

= VREF (1 + –––)

V+

Figure 6. Adjusting the Main Output Voltage

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