Rainbow Electronics MAX799 - Manual (Page 17)

MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
______________________________________________________________________________________ 17
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 V
REF , as shown in Figure 7. For best
accuracy, this second reference voltage should be
much higher than V
REF . 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
FB
R4
R5
MAIN
OUTPUT
REMOTE
SENSE
LINES DH
DL
V OUT
WHERE V
REF
(NOMINAL) = 2.505V
= V REF
(
1 + –––
) R4
R5
V+
Figure 6. Adjusting the Main Output Voltage