MAX8550-MAX8551 MAXIM [Maxim Integrated Products], MAX8550-MAX8551 Datasheet - Page 13

• Manufacturer Part Number: MAX8550-MAX8551
• Description: Integrated DDR Power-Supply Solutions for Desktops, Notebooks, and Graphic Cards
• Manufacturer: MAXIM [Maxim Integrated Products]

2) The inductor ripple-current operating point remains
The on-time one-shot has good accuracy at the operat-
ing points specified in the Electrical Characteristics
table (approximately ±12.5% at 600kHz and 450kHz,
and ±10% at 200kHz and 300kHz). On-times at operat-
ing points far removed from the conditions specified in
the Electrical Characteristics table can vary over a
wider range. For example, the 600kHz setting typically
runs approximately 10% slower with inputs much
greater than 5V due to the very short on-times required.
The constant on-time translates only roughly to a con-
stant switching frequency. The on-times guaranteed in
the Electrical Characteristics table are influenced by
resistive losses and by switching delays in the high-
side MOSFET. Resistive losses, which include the
inductor, both MOSFETs, the output capacitor’s ESR,
and any PC board copper losses in the output and
ground, tend to raise the switching frequency as the
load increases. The dead-time effect increases the
effective on-time, reducing the switching frequency as
one or both dead times are added to the effective on-
time. The dead time occurs only in PWM mode (SKIP =
V
when the inductor current reverses at light or negative
load currents. With reversed inductor current, the induc-
tor’s EMF causes LX to go high earlier than normal,
extending the on-time by a period equal to the DH-rising
dead time. For loads above the critical conduction point,
where the dead-time effect is no longer a factor, the
actual switching frequency is:
where V
in the inductor discharge path, including the synchro-
nous rectifier, the inductor, and any PC board resis-
tances; V
charging path, including the high-side switch (Q1 in the
Typical Applications Circuit), the inductor, and any PC
board resistances, and t
the On-Time One-Shot (TON) section.
In skip mode (SKIP = GND), an inherent automatic
switchover to PFM takes place at light loads (Figure 2).
This switchover is affected by a comparator that trun-
cates the low-side switch on-time at the inductor cur-
rent’s zero crossing. The zero-crossing comparator
DD
relatively constant, resulting in an easy design
methodology and predictable output voltage ripple.
) and during dynamic output-voltage transitions
DROP1
DROP2
Integrated DDR Power-Supply Solutions for
f
SW
Automatic Pulse-Skipping Mode
is the sum of the parasitic voltage drops
Desktops, Notebooks, and Graphic Cards
is the sum of the resistances in the
______________________________________________________________________________________
=
t
ON IN
V
ON
OUT
(
V
is the one-shot on-time (see
+
+
V
V
DROP
DROP
(
1
SKIP = GND)
2
)
differentially senses the inductor current across the
synchronous-rectifier MOSFET (Q2 in the Typical
Applications Circuit, Figure 8). Once V
drops below 5% of the current-limit threshold (2.5mV
for the default 50mV current-limit threshold), the com-
parator forces DL low (Figure 1). This mechanism caus-
es the threshold between pulse-skipping PFM and
nonskipping PWM operation to coincide with the
boundary between continuous and discontinuous
inductor-current operation (also known as the critical
conduction point). The load-current level at which
PFM/PWM crossover occurs, I
half the peak-to-peak ripple current, which is a function
of the inductor value (Figure 2). This threshold is rela-
tively constant, with only a minor dependence on the
input voltage (V
where K is the on-time scale factor (see Table 1). For
example, in the Typical Applications Circuit of Figure 8
(K = 1.7µs, V
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 can 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.
Table 1. Approximate K-Factor Errors
TON SETTING
(TON = OPEN)
(TON = AV
(TON = GND)
(TON = REF)
200
300
450
600
I
LOAD SKIP
⎛
⎜
⎝
2 5
DD
.
(
)
2
OUT
V
×
IN
×
TYPICAL
FACTOR
)
1
):
µ
= 2.5V, V
1 7
(µs)
5.0
3.3
2.2
1.7
=
K-
H
.
µ
⎛
⎜
⎝
s
V
OUT
⎞
⎟
⎠
⎛
⎜
⎝
K-FACTOR
12
2
ERROR
IN
L
V
±12.5
±12.5
×
(%)
±10
±10
12
= 12V, and L = 1µH), the
- .5V
K
LOAD(SKIP)
V
2
⎞
⎟
⎠
⎛
⎜
⎝
V
IN
⎞
⎟ =
⎠
(h = 1.5, SEE THE
MINIMUM V
PERFORMANCE
V
-
1 68
IN
V
V
.
DROPOUT
SECTION)
OUT
, is equal to
OUT
PGND
3.15
3.47
4.13
5.61
A
= 2.5V
⎞
⎟
⎠
IN
- V
AT
13
LX