MAX1652 Datasheet, PDF (16/28 Pages) Maxim Integrated Products – High-Efficiency, PWM, Step-Down DC-DC Controllers in 16-Pin QSOP

High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
+5V BATTERY
VL SUPPLY INPUT
MAX1652
VL
MAX1653
VL MAX1654
BST
MAX1655
LEVEL
DH
TRANSLATOR
LX
PWM
VL
DL
Figure 5. Boost Supply for Gate Drivers
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.
Current-Limiting and
Current-Sense Inputs (CSH and CSL)
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.
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.
Oscillator Frequency and
Synchronization (SYNC Pin)
The SYNC input controls the oscillator frequency.
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.
300kHz operation optimizes the application circuit for
component size and cost. 150kHz operation provides
increased efficiency and improved low-duty factor
operation (see Dropout Operation section).
Dropout Operation
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-
put voltage (V_OUT) drops out of regulation without the
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.
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%.
Low-Noise Mode (SKIP Pin)
The low-noise mode (SKIP = high) is useful for minimiz-
ing RF and audio interference in noise-sensitive appli-
cations such as audio-equipped systems, cellular
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.
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 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
16 ______________________________________________________________________________________