5
Date: 6/7/06 Rev B SP6648 Ultra-low Quiescent Current, High Efficiency Boost Regulator © 2006 Sipex Corporation
At light loads (as shown in plot A in Figure 1)
the charge cycle will last the maximum value for
t
ON
: For a 1V battery this would be as follows:
t
ON
= K
ON
/ V
BATT
= 4.0Vμs / 1V = 4.0μs
The current built up in the coil during the charge
cycle gets fully discharged in the discontinuous
conduction mode (DCM). When the current in
the coil has reached zero, the synchronous rec-
tifier switch is opened and the voltage across the
coil (from V
BATT
to LX) is shorted internally to
eliminate inductive ringing.
With increasing load (as shown in plot B in
Figure 1) this inductor damping time becomes
shorter, because the output will quickly drop
below its regulation point due to heavier load. If
the load current increases further, the SP6648
enters continuous conduction mode (CCM)
where there is always current flowing in the
inductor. The charge time remains at maximum
t
ON
as long as the inductor peak current limit is
not reached as shown in plot C in Figure 1. The
inductor peak current limit can be programmed
by tying a resistor R
LIM
from the R
LIM
pin to
ground where:
I
PEAK
= 1600 / R
LIM
When the peak current limit is reached the
charge time is short-cycled.
In plot D of Figure 1, the switch current reaches
the peak current limit during the charge period
which ends the charge cycle and starts the dis-
charge cycle. However, full load is not yet
achieved because at the end of the minimum
discharge time the output was still within regu-
lation.
Maximum load is reached when this discharge
time has shrunk to the minimum allowed value
T
OFF
as shown in Plot E of Figure 1.
Inductor Current vs. Load
Ton Max. Toff Min.
E. Iripple=Toff* (Vo
- Vi)/L
llim
llim
llim
llim
llim
Ton Max.
Ton Max.
Ton Max.
Ton Max.
Toff Min.
Toff Min.
Toff Min.
Toff Min.
D. Toff*= (Vo
- Vi)/L<Iripple<Ton*Vi/L
C. Iripple=Ton*Vi/L
B. Iripple=Ton*Vi/L
A. Iripple=Ton*Vi/L
E
D
C
B
A
Figure 1. Inductor Current vs. Load
THEORY OF OPERATION: Continued
