SP6648 Datasheet, PDF (5/16 Pages) Sipex Corporation – Ultra-low Quiescent Current, High Efficiency Boost Regulator

THEORY OF OPERATION: Continued
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 (discontinuous con-
duction mode DCM) When the current in the
coil has reached zero the synchronous rectifier
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
Inductor Current vs. Load
Ton Max.
Toff Min.
E. Iripple=Toff* (Vo - Vi)/L
llim
E
Ton Max.
Toff Min.
D. Toff*= (Vo - Vi)/L<Iripple<Ton*Vi/L
llim
D
Ton Max. Toff Min.
C. Iripple=Ton*Vi/L
llim
C
Ton Max. Toff Min. _{B. Iripple=Ton*Vi/L}
llim
B
Ton Max. Toff Min.
A. Iripple=Ton*Vi/L
llim
A
Figure 1. Inductor Current vs. Load
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.
Date: 7/19/04
SP6648 Ultra-low Quiescent Current, High Efficiency Boost Regulator
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© Copyright 2004 Sipex Corporation