MAX1540A Datasheet, PDF (25/49 Pages) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

Dual Step-Down Controllers with Saturation
Protection, Dynamic Output, and Linear Regulator
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
V_CC
Open
REF
GND
OUT1 MODE
Forced PWM
Forced PWM
Pulse skipping
Pulse skipping
OUT2 MODE
Forced PWM
Pulse skipping
Forced PWM
Pulse skipping
ing the high-side switch, inductor, and PC board resis-
tances; and t_ON is the on-time calculated by the
MAX1540A/MAX1541.
Light-Load Operation (SKIP)
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:
I_LOAD(SKIP)
≈
⎛
⎝⎜
V_OUTK
2L
⎞
⎠⎟
⎛
⎝⎜
V_IN
- V_OUT
V_IN
⎞
⎠⎟
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:
⎛
⎝⎜
2.5V
2×
× 3.0μs
4.3μH
⎞
⎠⎟
⎛
⎝⎜
12V - 2.5V
12V
⎞
⎠⎟
=
0.69A
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 MAX1540A/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
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