Datasheet

LTC3520
14
3520fa
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:
fkHz
Rk
T
()
,
()
=
54 000
Ω
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 fi ltering and prevent glitching during slow
transitions through the threshold region. The gain block
is not disabled by the undervoltage lockout. This allows
the uncommitted amplifi er 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 amplifi er 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 (SD3) 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.
Figure 2. Gain Block Used as a Comparator
Figure 3. Gain Block Confi gured as an LDO
OPERATION
V
OUT
2.5V
200mA
3.3V
33pF
169k
76.8k
4.7
μF
3520 F05
LTC3520
A
IN
A
OUT
V
OUT
PGOOD
750k
402k
3520 F02
LTC3520
A
IN
A
OUT
A
IN
A
OUT
V
BAT
LBO
2.49M
330pF
806k
LTC3520