SP6649 Datasheet, PDF (5/9 Pages) Sipex Corporation – Ultra-low Quiescent Current, High Efficiency Boost DC-DC Regulator

At light loads (plot A in Figure 3) the charge
cycle will take t_{ON, MAX} µs. For a 1V battery this
would be:
t_{ON, MAX} = K_ON / V_BATT = 3.3Vµs / 1V = 3.3µs
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 (plot B in Figure 3) this
inductor damping time becomes shorter (be-
cause the output will drop quicker below its
regulation point due to the heavier load) up to
the point where it becomes zero. If the load
increases further the SP6649 enters continuous
conduction mode (CCM) where there is always
current in the inductor. The charge time is still
t_ON,MAX as long as the inductor peak current
limit is not reached (plot C in Figure 3). the
inductor peak current limit can be programmed
by trying a resistor R_LIM from the R_LIM pin to
ground where:
I_PEAK = 1400 / R_LIM
with a maximum recommended I_PEAK of 1.2A
(or a minimum R_LIM of 1.17K ).
When the peak current limit is reached the
charge time is short-cycled.
Inductor Current vs. Load
llim
Ton Max.
Toff Min.
E. Iripple=Toff* (Vo - Vi)/L
Ton Max.
llim
Toff Min.
D. Toff*= (Vo - Vi)/L<Iripple<Ton*Vi/L
llim
Ton Max. Toff Min.
C. Iripple=Ton*Vi/L
llim
Ton Max. Toff Min.
B. Iripple=Ton*Vi/L
llim
Ton Max. Toff Min.
A. Iripple=Ton*Vi/L
Figure 3. Inductor Current vs. Load
In (plot D of Figure 3) the current reaches the
peak current limit during the charge cycle but
full load is still not reached becuse at the end of
the minimum off-time V_OUT was still not below
its regulation point. Finally in plot E the maxi-
mum load is reached where the discharge time
has shrunk to its minimum allowed value
t_OFF,MIN.
Rev:001 Date:11/20/03
SP6649 Ultra-low Quiescent Current, High Efficiency Boost DC-DC Regulator
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© Copyright 2002 Sipex Corporation