LTC3527-1_15 Datasheet, PDF (12/22 Pages) Linear Technology – Dual 800mA/400mA, 1.2MHz/2.2MHz Synchronous Step-Up DC/DC Converters

LTC3527/LTC3527-1
OPERATION
output until it reaches the nominal regulation value, then
the LTC3527/LTC3527-1 transition to sleep mode where
the outputs are off and the LTC3527/LTC3527-1 consume
only 12μA of quiescent current from the higher of V_OUT1 or
V_OUT2. When the output voltage droops slightly, switching
resumes. This maximizes efficiency at very light loads by
minimizing switching and quiescent current losses. Burst
Mode output voltage ripple, which is typically 1% peak-to-
peak, can be reduced by using more output capacitance
(10μF or greater), or with a small capacitor (15pF) con-
nected between V_OUT1 (V_OUT2) and FB1 (FB2).
If either load current increases, the LTC3527/LTC3527-1
will automatically leave Burst Mode operation. Note that
larger output capacitor values may cause this transition
to occur at lighter loads. Once the LTC3527/LTC3527-1
have left Burst Mode operation and returned to normal
operation, they will remain there until both output loads
are reduced below the burst threshold current.
Burst Mode operation is inhibited during start-up and soft-
start and until both V_OUT1 and V_OUT2 are at least 0.24V
greater than V_IN if neither channel is in shutdown.
When the MODE pin is high, LTC3527/LTC3527-1 feature
continuous PWM fixed frequency operation at 1.2MHz
(FSEL = Low) or 2.2MHz (FSEL = High). At very light
loads, the LTC3527/LTC3527-1 will exhibit pulse-skipping
operation.
Single Cell to 5V Step-Up Applications
Due to the high inductor current slew rate in applica-
tions boosting to 5V from a single-cell (alkaline, NiCd
or NiMH), the LTC3527/LTC3527-1 may not enter Burst
Mode operation at input voltages below 1.5V in a 2.2MHz
application (FSEL = high). For a single-cell to 5V appli-
cation requiring Burst Mode 1.2MHz operation, (FSEL =
low) is recommended. Refer to the Typical Performance
Characteristics for the Burst Mode thresholds for different
input and output voltages.
APPLICATIONS INFORMATION
V_IN > V_OUT Operation
The LTC3527/LTC3527-1 will maintain output voltage
regulation even when the input voltage is above one or
both of the desired output voltages. Note, all V_INS must
be common to support this mode of operation. Since
this mode is less efficient and will dissipate more power
in the LTC3527/LTC3527-1, the maximum output current
capability is limited in order to maintain an acceptable
junction temperature. When operating with V_IN > V_OUT
the power is defined by:
P_OUT = I_OUT ⎡⎣(V_IN + 1.5) − V_OUT ⎤⎦
To maintain a junction temperature below 125°C, the fol-
lowing formula must be adhered to:
(P_OUT1 + P_OUT2) 68°C/ W = 125 − T_A
where T_A is the ambient temperature.
Short-Circuit Protection
The LTC3527/LTC3527-1 output disconnect feature allows
an output short-circuit while maintaining a maximum in-
ternally set current limit. The converters also incorporate
internal features such as current limit foldback and thermal
shutdown for protection from an excessive overload or
short circuit. To reduce power dissipation under short-
circuit conditions, the peak switch current limit is reduced
to 500mA (typical) for converter 1 and 350mA (typical)
for converter 2 when V_OUT is less than 1V.
Schottky Diode
Although it is not required, adding a Schottky diode from
SW1 (SW2) to V_OUT1 (V_OUT2) will improve efficiency by
about 2%. Note that this defeats the output disconnect
and short-circuit protection features.
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