Rainbow Electronics MAX1813 - Manual (Page 17)

MAX1813
Dynamically-Adjustable, Synchronous Step-Down
Controller with Integrated Voltage Positioning
______________________________________________________________________________________ 17
easy design methodology and predictable output volt-
age ripple.
where K is set by the TON pin-strap connection, and
75mV is an approximation to accommodate for the
expected drop across the low-side MOSFET switch and
current-sense resistor (Table 3).
The on-time one-shot has good accuracy at the operat-
ing points specified in the Electrical Characteristics
table. On-times at operating points far removed from
the conditions specified can vary over a wide range.
For example, the 1000kHz setting will typically run
about 10% slower with inputs much greater than +5V,
due to the very short on-times required.
Although the on-time is set by TON, the input voltage,
and the output voltage, other factors also contribute to
the overall switching frequency. The on-time guaran-
teed in the Electrical Characteristics table is influenced
by switching delays in the external high-side MOSFET.
Resistive losses—including the inductor, both
MOSFETs, output capacitor ESR, and PC board copper
losses in the output and ground—tend to raise the
switching frequency at higher output currents. Switch
dead-time can increase the effective on-time, reducing
the switching frequency. This effect occurs only in
PWM mode (SKP/SDN = float) when the inductor cur-
rent reverses at light or negative load currents. With
reversed inductor current, the inductor’s EMF causes
LX to go high earlier than normal, extending the on-time
by a period equal to the DH-rising dead-time (26ns
typ).
When the controller operates in continuous
mode, the dead-time is no longer a factor, and the
actual switching frequency is:
where V DROP1 is the sum of the parasitic voltage drops
in the inductor discharge path, including synchronous
rectifier, inductor, and PC board resistances; V
DROP2 is
the sum of the resistances in the charging path, includ-
ing high-side switch, inductor, and PC board resis-
tances; and t
ON is the on-time calculated by the
MAX1813.
Automatic Pulse-Skipping Switchover
In skip mode (SKP/SDN = high, Table 4), an inherent
automatic switchover to PFM takes place at light
loads (Figure 3). This switchover is controlled 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 continuous and discontinuous
inductor-current operation. For a 7V to 24V input volt-
age range, this threshold is relatively constant, with
ƒ =
+
+ − SW
OUT DROP1
ON IN DROP1 DROP2
(V V )
t (V V V )
t
K(V 75mV)
V
ON
OUT
IN
=
+
TON SETTING
(kHz)
K-FACTOR
( µs)
APPROXIMATE
K-FACTOR ERROR
(%)
MINIMUM RECOMMENDED V BATT
AT V
OUT
= 1.4V (V)
200 4.9 ±9 1.8
300 3.3 ±10 2.0
600 1.8 ±13 2.9
1000 1.05 ±13 3.5
Table 3. Approximate K-Factor Errors
INDUCTOR CURRENT
I LOAD = I PEAK /2
ON-TIME 0 TIME
I PEAK
L
V BATT - V OUT
∆i
∆t
=
Figure 3. Pulse-Skipping/Discontinuous Crossover Point