LTC3520 Datasheet, PDF (14/24 Pages) Linear Technology – Synchronous 1A Buck-Boost and 600mA Buck Converters

LTC3520
OPERATION
COMMON FUNCTIONS
Oscillator
The buck-boost and buck converters operate from a com-
mon internal oscillator. The switching frequency for both
converters is set by the value of an external resistor, R_T,
located between the R_T pin and ground according to the
following equation:
f(kHz) = ^54,000
R_T(kΩ)
Gain Block
The LTC3520 contains a gain block (pins A_IN and A_OUT)
that can be used as a low battery indicator or power-good
comparator for either the buck or buck-boost output volt-
age. Typical circuits for these applications are shown in
Figure 2. A small-valued capacitor can be added from A_OUT
to GND to provide filtering and prevent glitching during slow
transitions through the threshold region. The gain block
is not disabled by the undervoltage lockout. This allows
the uncommitted amplifier to be utilized as a low battery
indicator down to a supply voltage of 1.6V typically.
The A_OUT pin is not an open-drain output. Rather, it is a
push-pull output that can both sink and source current.
The uncommitted amplifier is internally powered by the
higher of either the SV_IN or V_OUT1 voltages. This restricts
the maximum voltage on the A_OUT pin to either the input
supply voltage or the buck-boost output voltage, which-
ever is larger.
Alternatively, the gain block can be utilized as an LDO
with the addition of an external PNP as shown in
Figure 3. The LDO is convenient for applications requiring
a third output (possibly a low current 2.5V or a quiet 3V
supply). An external PMOS can be used in place of the PNP,
but a much larger output capacitor is required to ensure
stability at light load. The gain block has an independent
shutdown pin (^⎯S^⎯D^⎯3) and should be disabled when not in
use to reduce quiescent current.
Thermal Shutdown
If the die temperature exceeds 150ºC both converters
will be disabled. All power devices will be turned off and
all switch nodes will be high impedance. The soft-start
circuits for both converters are reset during thermal
shutdown to provide a smooth recovery once the over-
temperature condition is eliminated. Both converters will
restart (if enabled) when the die temperature drops to
approximately 140ºC.
Undervoltage Lockout
If the supply voltage decreases below 2V (typical) then
both converters will be disabled and all power devices will
be turned off. The soft-start circuits for both converters
are reset during undervoltage lockout to provide a smooth
restart once the input voltage rises above the undervoltage
lockout threshold.
3.3V
LBO
V_BAT
A_OUT
2.49M
LTC3520
A_IN
806k
PGOOD
V_OUT
330pF
750k
402k
A_OUT
LTC3520
A_IN
3520 F02
V_OUT
2.5V
200mA
4.7µF
169k
76.8k
A_OUT
LTC3520
33pF
A_IN
3520 F05
Figure 2. Gain Block Used as a Comparator
Figure 3. Gain Block Configured as an LDO
3520f
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