Datasheet

LTC3625/LTC3625-1
9
3625f
operaTion
Single Inductor Operation
With the CTL pin tied to V
IN
the LTC3625/LTC3625-1 will
operate in single inductor mode. In this mode the same
inductor serves in the power path for both the buck and
the boost converters. Thus, the buck converter and boost
converter will never run simultaneously.
Under certain conditions with a single inductor, a small
amount of current can flow from the supercapacitors to V
IN
when the boost charger is active. A 25mA load is required
on V
IN
to prevent the V
IN
supply from being pumped to
a higher voltage while the boost is active. This minimum
load is not needed in the two inductor application and it
is also not needed when the charger is disabled.
A typical charge cycle for a fully discharged capacitor stack
will proceed as follows:
1.
The
buck converter will turn on and regulate its output
current ramping hysteretically between 1.1 • I
BUCK
and
0.9 • I
BUCK
until the V
MID(GOOD)
threshold is met (1.35V
typical).
2. Once
the V
MID(GOOD)
threshold is reached, the boost
converter will turn on and regulate its input current
ramping hysteretically between 2.12A and 1.88A until
V
MID
falls below the V
MID(GOOD)
hysteresis threshold
(1.2V typical).
3. Phases
1 and 2 will alternate until V
OUT
is approximately
2.4V. When V
TOP
(equal to V
OUT
– V
MID
) is approximately
50mV > V
MID
, the boost regulator will turn off and the
buck regulator will turn on. Likewise, when V
MID
is
approximately 50mV > V
TOP
, the boost regulator will
turn on and the buck regulator will turn off.
4. Phase
3 will continue until V
OUT
has reached its pro-
grammed output voltage. Once this happens, the part
will enter sleep mode and only minimal power will be
consumed (see the Electrical Characteristics table).
5.
If the supercapacitors’ self discharge or an external load
cause the output to drop by more than 135mV (typical),
then the LTC3625/LTC3625-1 will exit sleep mode and
begin charging the appropriate supercapacitor.
In all cases whenever either of the converters is shut
down, it will switch to its appropriate discharge phase
(NMOS on for the buck and PMOS on for the boost) until
the inductor current reaches 0mA. This optimizes charge
delivery to the output capacitors.
Charge time is dependant on the programmed buck output
current as well as the value of the supercapacitors being
charged. For estimating charge profiles in the single induc-
tor application, see the Typical Performance Characteristics
graph Charge Time vs R
PROG
.
The effective average V
OUT
referred charge current can
be approximated as:
I I
A
I A
CHARGE BUCK BOOST
BUCK
≅
+
0 5
2
2
. • • •ε
where ε
BOOST
is the boost converter efficiency, which is
typically about 85% (see the Typical Performance Char-
acteristics graph Boost Efficiency vs V
TOP
).
Seen another way, this is the maximum steady-state load
the part can support without losing V
OUT
regulation.
Dual Inductor Operation
With the CTL pin tied to GND, the LTC3625/LTC3625-1
will operate in dual inductor mode. In this mode two
inductors will serve as the power path for the buck and
the boost converters. This will allow both the buck and
the boost converter to run simultaneously. As a result, the
total charge time will be greatly reduced at the cost of an
additional board component.
A typical charge cycle for a fully discharged capacitor stack
will proceed as follows:
1. The buck converter will turn on and regulate its output
current ramping hysteretically between 1.1 • I
BUCK
and 0.9 • I
BUCK
until the V
MID(GOOD)
threshold is met
(1.35V typical).
2. Once the V
MID(GOOD)
threshold is reached, the boost
converter will turn on and regulate its input current
ramping hysteretically between 2.12A and 1.88A. The
buck converter will continue to run at the same time. In
some cases (I
BUCK
~ <1A) the boost converter’s input
current will exceed the current delivered to C
BOT
; even
though the buck converter is running, charge will be
removed and V
MID
may decrease. Thus, if V
MID
falls
below the V
MID(GOOD)
hysteresis threshold, the boost