Rainbow Electronics MAX1715 User Manual
Page 14
MAX1715
Ultra-High Efficiency, Dual Step-Down
Controller for Notebook Computers
14
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nominal frequency setting (200kHz, 300kHz, 420kHz, or
540kHz), while the on-times for side 2 are set 15%
lower than nominal. This is done to prevent audio-fre-
quency “beating” between the two sides, which switch
asynchronously for each side:
On-Time = K (V
OUT
+ 0.075V) / V
IN
where K is set by the TON pin-strap connection and
0.075V is an approximation to accommodate for the
expected drop across the low-side MOSFET switch.
One-shot timing error increases for the shorter on-time
settings due to fixed propagation delays; it is approxi-
mately ±12.5% at 540kHz and 420kHz nominal settings
and ±10% at the two slower settings. This translates to
reduced switching-frequency accuracy at higher fre-
quencies (Table 5). Switching frequency increases as a
function of load current due to the increasing drop
across the low-side MOSFET, which causes a faster
inductor-current discharge ramp. The on-times guaran-
teed in the Electrical Characteristics are influenced by
switching delays in the external high-side power MOS-
FET.
Two external factors that influence switching-frequency
accuracy are resistive drops in the two conduction
loops (including inductor and PC board resistance) and
the dead-time effect. These effects are the largest con-
tributors to the change of frequency with changing load
current. The dead-time effect increases the effective
on-time, reducing the switching frequency as one or
both dead times. It occurs only in PWM mode (SKIP =
high) when the inductor current reverses at light or neg-
ative load currents. With reversed inductor current, the
inductor’s EMF causes LX to go high earlier than nor-
mal, extending the on-time by a period equal to the
low-to-high dead time.
For loads above the critical conduction point, the actual
switching frequency is:
where V
DROP
1 is the sum of the parasitic voltage drops
in the inductor discharge path, including synchronous
rectifier, inductor, and PC board resistances; VDROP2
is the sum of the resistances in the charging path; and
t
ON
is the on-time calculated by the MAX1715.
Automatic Pulse-Skipping Switchover
In skip mode (SKIP low), an inherent automatic
switchover to PFM takes place at light loads. This
switchover is effected by a comparator that truncates
the low-side switch on-time at the inductor current’s
zero crossing. 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). For a
battery range of 7V to 24V, this threshold is relatively
constant, with only a minor dependence on battery volt-
age.
where K is the on-time scale factor (Table 5). The load-
current level at which PFM/PWM crossover occurs,
I
LOAD(SKIP)
, is equal to 1/2 the peak-to-peak ripple cur-
rent, which is a function of the inductor value (Figure 4).
For example, in the standard application circuit with
V
OUT1
= 2.5V, V
IN
= 15V, and K = 2.96µs (see Table
5), switchover to pulse-skipping operation occurs at
I
LOAD
= 0.7A or about 1/6 full load. The crossover point
occurs at an even lower value if a swinging (soft-satura-
tion) inductor is used.
The switching waveforms may appear noisy and asyn-
chronous when light loading causes pulse-skipping
I
K V
2L
V
- V
V
LOAD(SKIP)
OUT_
IN
OUT
IN
≈
×
f
V
V
t
V
V
OUT
DROP
ON
IN
DROP
=
+
+
(
)
1
2
Good operating point for
compound buck designs
or desktop circuits.
+5V input
540
420
3-cell Li+ notebook
Useful in 3-cell systems
for lighter loads than the
CPU core or where size is
key.
Considered mainstream
by current standards.
4-cell Li+ notebook
300
200
4-cell Li+ notebook
Use for absolute best
efficiency.
COMMENTS
TYPICAL
APPLICATION
NOMINAL
FREQUENCY
(kHz)
Table 4. Frequency Selection Guidelines
Table 5. Approximate K-Factor Errors
TON
SETTING
MIN V
IN
AT V
OUT
= 2V (V)
SIDE 1 K
FACTOR
(µs)
V
CC
2.6
4.24
OPEN
2.9
2.96
REF
3.2
2.08
GND
3.6
1.63
APPROX
K-FACTOR
ERROR (%)
±10
±10
±12.5
±12.5
SIDE 2 K
FACTOR
(µs)
5.81
4.03
2.81
2.18