Rainbow Electronics MAX1541 - Manual (Page 26)

MAX1540/MAX1541
Forced-PWM Mode
The low-noise forced-PWM mode disables the zero-
crossing comparator, which controls the low-side
switch on-time. This forces the low-side gate-drive
waveform to be constantly the complement of the high-
side gate-drive waveform, so the inductor current
reverses at light loads while DH_ maintains a duty fac-
tor of V
OUT_
/ V
IN
. The benefit of forced-PWM mode is
to keep the switching frequency fairly constant.
However, forced-PWM operation comes at a cost: the
no-load 5V bias current remains between 4mA to
40mA, depending on the external MOSFETs and
switching frequency.
Forced-PWM mode is most useful for reducing audio-
frequency noise, improving load-transient response,
and providing sink-current capability for dynamic out-
put-voltage adjustment. The MAX1541 uses forced-
PWM operation during all dynamic output-voltage
transitions (GATE transition detected) in order to ensure
fast, accurate transitions. Since forced-PWM operation
disables the zero-crossing comparator, the inductor
current reverses under light loads, quickly discharging
the output capacitors. FBLANK determines how long
the MAX1541 maintains forced-PWM operation—typi-
cally 220µs (FBLANK = V
CC
), 140µs (FBLANK = open
or GND), or 65µs (FBLANK = REF).
Current-Limit Protection (ILIM_)
Valley Current Limit
The current-limit circuit employs a unique “valley” cur-
rent-sensing algorithm that uses a current-sense resistor
between CSP_ and CSN_ as the current-sensing ele-
ment (Figure 1). If the magnitude of the current-sense
signal is above the valley current-limit threshold, the
PWM controller is not allowed to initiate a new cycle
Dual Step-Down Controllers with Saturation
Protection, Dynamic Output, and Linear Regulator
26 ______________________________________________________________________________________
Figure 3. MAX1540/MAX1541 PWM-Controller Functional Diagram
VALLEY CURRENT
LIMIT
ON-TIME
COMPUTE
V+
TON
ERROR
AMP
SATURATION
LIMIT
S
R
Q
S
R
Q DH DRIVER
1-SHOT
TRIG Q
t OFF(MIN)
1-SHOT
TRIG Q
t ON
OUT_ ZERO
CROSSING
FAULT
PROTECTION
DL DRIVER
INT REF_
INT FB_
SLOPE COMP CC1
*MAIN MAX1541
CONTROLLER (OUT1) ONLY
CC1
7R
R
G m = 80
A
V
Figure 4. Pulse-Skipping/Discontinuous Crossover Point
INDUCTOR CURRENT
I LOAD = I PEAK /2
ON-TIME 0 TIME
I
PEAK
L
V IN - V OUT
∆I
∆t
=