Rainbow Electronics MAX1541 - Manual (Page 25)

Light-Load Operation (
S SK KIIP P
)
The four-level SKIP input selects light-load, pulse-skip-
ping operation by independently enabling or disabling
the zero-crossing comparator for each controller (Table
4). When the zero-crossing comparator is enabled, the
controller forces DL_ low when the current-sense inputs
detect zero inductor current. This keeps the inductor
from discharging the output capacitors and forces the
controller to skip pulses under light-load conditions to
avoid overcharging the output. When the zero-crossing
comparator is disabled, the controller maintains PWM
operation under light-load conditions (see the Forced-
PWM Mode section).
Automatic Pulse-Skipping Mode
In skip mode, an inherent automatic switchover to PFM
takes place at light loads (Figure 3). This switchover is
affected by a comparator that truncates the low-side
switch on-time at the inductor current’s zero crossing.
The zero-crossing comparator differentially senses the
inductor current across the current-sense inputs (CSP_
to CSN_). Once V
CSP_ - V CSN_ drops below 5% of the
current-limit threshold (2.5mV for the default 50mV cur-
rent-limit threshold), the comparator forces DL_ low
(Figure 3). This mechanism causes the threshold
between pulse-skipping PFM and nonskipping PWM
operation to coincide with the boundary between con-
tinuous and discontinuous inductor-current operation
(also known as the “critical-conduction” point). The
load-current level at which PFM/PWM crossover
occurs, I
LOAD(SKIP)
, is equal to half the peak-to-peak
ripple current, which is a function of the inductor value
(Figure 4). This threshold is relatively constant, with
only a minor dependence on battery voltage:
where K is the on-time scale factor (Table 3). For exam-
ple, in the MAX1541 Standard Application Circuit
(Figure 12) (K = 3.0µs, V
OUT2
= 2.5V, V
IN
= 12V, and L
= 4.3µH), the pulse-skipping switchover occurs at:
The crossover point occurs at an even lower value if a
swinging (soft-saturation) inductor is used. The switch-
ing waveforms may appear noisy and asynchronous
when light loading causes pulse-skipping operation,
but this is a normal operating condition that results in
high light-load efficiency. Trade-offs in PFM noise vs.
light-load efficiency are made by varying the inductor
value. Generally, low inductor values produce a broad-
er efficiency vs. load curve, while higher values result in
higher full-load efficiency (assuming that the coil resis-
tance remains fixed) and less output voltage ripple.
Penalties for using higher inductor values include larger
physical size and degraded load-transient response
(especially at low input-voltage levels).
DC-output accuracy specifications refer to the thresh-
old of the error comparator. When the inductor is in
continuous conduction, the MAX1540/MAX1541 regu-
late the valley of the output ripple, so the actual DC out-
put voltage is higher than the trip level by 50% of the
output ripple voltage. In discontinuous conduction
(I
OUT < I LOAD(SKIP) ), the output voltage has a DC regu-
lation level higher than the error-comparator threshold
by approximately 1.5% due to slope compensation.
.
.
2.5V 3 s
2 4.3 H
12V - 2.5V
12V
Ч
Ч

 
 
 

=
0
0 69
µ
µ
A
I
LOAD(SKIP)
OUT IN OUT
IN
V K
2L
V - V
V
≈ 


 
 

MAX1540/MAX1541
Dual Step-Down Controllers with Saturation
Protection, Dynamic Output, and Linear Regulator
______________________________________________________________________________________ 25
Table 3. Approximate K-Factor Errors
CONTROLLER 1 (OUT1) CONTROLLER 2 (OUT2)
NOMINAL TON
SETTING (kHz) K-FACTOR
ERROR (%)
TYPICAL
K-FACTOR
(µs)
MINIMUM V IN AT
V OUT1 = 1.8V*
(V)
TYPICAL
K-FACTOR
(µs)
MINIMUM V IN AT
V OUT2 = 2.5V*
(V)
200kHz (TON = V CC ) ±10 4.5 (235kHz) 2.28 6.2 (170kHz) 2.96
300kHz (TON = open) ±10 3.0 (345kHz) 2.52 4.1 (255kHz) 3.18
420kHz (TON = REF) ±12.5 2.2 (485kHz) 2.91 3.0 (355kHz) 3.48
540kHz (TON = GND) ±12.5 1.7 (620kHz) 3.42 2.3 (460kHz) 3.87
*See the Step-Down Converter Dropout Performance section (h = 1.5 and worst-case K-factor value used).
Table 4.
SKIP Configuration Table
SKIP
OUT1 MODE OUT2 MODE
V CC Forced PWM Forced PWM
Open Forced PWM Pulse skipping
REF Pulse skipping Forced PWM
GND Pulse skipping Pulse skipping