Rainbow Electronics MAX1718 User Manual
Page 30
MAX1718
Notebook CPU Step-Down Controller for Intel
Mobile Voltage Positioning (IMVP-II)
30
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Forced-PWM Mode section). However, processor sus-
pend currents can be low enough that Skip mode oper-
ation provides a real benefit.
In the circuit of Figure 17, SKP/SDN remains biased at
2V in every state except Suspend and Shutdown. In
addition, upon entering Suspend (SUS going high) the
pin remains at 2V for about 200µs before it eventually
goes high. This causes the MAX1718 to remain in PWM
mode long enough to correctly complete the negative
output voltage transition to the Suspend state voltage.
When SKP/SDN goes high, the MAX1718 enters its low-
quiescent-current Skip mode.
Dropout Performance
The output voltage adjust range for continuous-conduc-
tion operation is restricted by the nonadjustable 500ns
(max) minimum off-time one-shot (375ns max at
1000kHz). For best dropout performance, use the slower
(200kHz) on-time settings. When working with low input
voltages, the duty-factor limit must be calculated using
worst-case values for on- and off-times. Manufacturing
tolerances and internal propagation delays introduce
an error to the TON K-factor. This error is greater at
higher frequencies (Table 2). Also, keep in mind that
transient response performance of buck regulators
operated close to dropout is poor, and bulk output
capacitance must often be added (see the VSAG equa-
tion in the Design Procedure section).
The absolute point of dropout is when the inductor cur-
rent ramps down during the minimum off-time (
∆I
DOWN
)
as much as it ramps up during the on-time (
∆I
UP
). The
ratio h =
∆I
UP
/
∆I
DOWN
is an indicator of ability to slew
the inductor current higher in response to increased
load, and must always be greater than 1. As h
approaches 1, the absolute minimum dropout point, the
inductor current will be less able to increase during
each switching cycle and V
SAG
will greatly increase
unless additional output capacitance is used.
A reasonable minimum value for h is 1.5, but this may
be adjusted up or down to allow tradeoffs between
V
SAG
, output capacitance, and minimum operating
voltage. For a given value of h, the minimum operating
voltage can be calculated as:
where V
DROP1
and V
DROP2
are the parasitic voltage
drops in the discharge and charge paths, respectively
(see the On-Time One-Shot (TON) section), T
OFF(MIN)
is
from the Electrical Characteristics tables, and K is taken
from Table 2. The absolute minimum input voltage is cal-
culated with h = 1.
If the calculated V
IN(MIN)
is greater than the required
minimum input voltage, then operating frequency must
be reduced or output capacitance added to obtain an
acceptable V
SAG
. If operation near dropout is anticipat-
ed, calculate V
SAG
to be sure of adequate transient
response.
Dropout Design Example:
V
OUT
= 1.6V
fsw = 550kHz
K = 1.8µs, worst-case K = 1.58µs
T
OFF(MIN)
= 500ns
V
DROP1
= V
DROP2
= 100mV
h = 1.5
V
IN(MIN)
= (1.6V + 0.1V) / (1-0.5µs x 1.5/1.58µs) + 0.1V
- 0.1V = 3.2V
Calculating again with h = 1 gives the absolute limit of
dropout:
V
IN(MIN)
= (1.6V + 0.1V) / (1-1.0
✕
0.5µs/1.58µs) - 0.1V
+ 0.1V = 2.5V
Therefore, V
IN
must be greater than 2.5V, even with very
large output capacitance, and a practical input voltage
with reasonable output capacitance would be 3.2V.
Adjusting V
OUT
with a Resistor-Divider
The output voltage can be adjusted with a resistor-
divider rather than the DAC if desired (Figure 18). The
drawback is that the on-time doesn’t automatically
receive correct compensation for changing output voltage
levels. This can result in variable switching frequency
as the resistor ratio is changed, and/or excessive
switching frequency. The equation for adjusting the output
voltage is:
where V
FB
is the currently selected DAC value. In resis-
tor-adjusted circuits, the DAC code should be set as
close as possible to the actual output voltage in order
to minimize the shift in switching frequency.
One-Stage (Battery Input) vs. Two-Stage
(5V Input) Applications
The MAX1718 can be used with a direct battery connec-
tion (one stage) or can obtain power from a regulated 5V
supply (two stage). Each approach has advantages,
V
V
R
R
OUT
FB
=
+
1
1
2
V
V
V
T
x h
K
V
V
IN MIN
OUT
DROP
OFF MIN
DROP
DROP
(
)
(
)
=
+
(
)
−
+
−
1
2
1
1