Hig h -Effic ie n c y, P WM, S t e p -Do w n
DC-DC Co n t ro lle rs in 1 6 -P in QS OP
Os c illa t o r Fre q u e n c y a n d
S yn c h ro n iza t io n (S YNC P in )
BATTERY
INPUT
+5V
VL SUPPLY
The SYNC inp ut c ontrols the os c illa tor fre q ue nc y.
Connecting SYNC to GND or to VL selects 150kHz
operation; connecting SYNC to REF selects 300kHz.
SYNC can also be used to synchronize with an external
5V CMOS clock generator. SYNC has a guaranteed
190kHz to 340kHz capture range.
VL
MAX1652
MAX1653
MAX1654
MAX1655
VL
BST
300kHz operation optimizes the application circuit for
component size and cost. 150kHz operation provides
inc re a s e d e ffic ie nc y a nd imp rove d low-d uty fa c tor
operation (see Dropout Operation section).
DH
LX
LEVEL
TRANSLATOR
PWM
VL
Dro p o u t Op e ra t io n
Dropout (low input-output differential operation) is en-
hanced by stretching the clock pulse width to increase
the maximum duty factor. The algorithm follows: if the out-
DL
put voltage (V ) drops out of regulation without the
OUT
current limit having been reached, the controller skips an
off-time period (extending the on-time). At the end of the
cycle, if the output is still out of regulation, another off-time
period is skipped. This action can continue until three off-
time periods are skipped, effectively dividing the clock
frequency by as much as four.
2–MAX165
Figure 5. Boost Supply for Gate Drivers
The typical PWM minimum off-time is 300ns, regardless
of the operating frequency. Lowering the operating fre-
quency raises the maximum duty factor above 98%.
an action that “boosts” the 5V gate-drive signal above
the battery voltage.
Ringing seen at the high-side MOSFET gate (DH) in
discontinuous-conduction mode (light loads) is a natur-
al operating condition caused by the residual energy in
the tank circuit formed by the inductor and stray capac-
itance at the switching node LX. The gate-driver nega-
tive rail is referred to LX, so any ringing there is directly
coupled to the gate-drive output.
Lo w -No is e Mo d e (S KIP P in )
The low-noise mode (SKIP = high) is useful for minimiz-
ing RF and audio interference in noise-sensitive appli-
c a tions s uc h a s a ud io-e q uip p e d s ys te ms , c e llula r
phones, RF communicating computers, and electro-
magnetic pen-entry systems. See the summary of oper-
ating modes in Table 3. SKIP can be driven from an
external logic signal.
Cu rre n t -Lim it in g a n d
Cu rre n t -S e n s e In p u t s (CS H a n d CS L)
The MAX1653 and MAX1655 can reduce interference
due to switching noise by ensuring a constant switch-
ing frequency regardless of load and line conditions,
thus concentrating the emissions at a known frequency
outside the system audio or IF bands. Choose an oscil-
lator frequency where harmonics of the switching fre-
quency don’t overlap a sensitive frequency band. If
necessary, synchronize the oscillator to a tight-toler-
ance external clock generator.
The current-limit circuit resets the main PWM latch and
turns off the high-side MOSFET switch whenever the
voltage difference between CSH and CSL exceeds
100mV. This limiting is effective for both current flow
directions, putting the threshold limit at ±100mV. The
tolerance on the positive current limit is ±20%, so the
external low-value sense resistor must be sized for
80mV/R1 to guarantee enough load capability, while
components must be designed to withstand continuous
current stresses of 120mV/R1.
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
For breadboarding purposes or very-high-current appli-
cations, it may be useful to wire the current-sense inputs
with a twisted pair rather than PC traces.
16 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
