LTC3520 Linear Technology, LTC3520 Datasheet - Page 14

• Manufacturer Part Number: LTC3520
• Description: Synchronous 1A Buck-Boost and 600mA Buck Converters
• Manufacturer: Linear Technology

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LTC3520
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
located between the R
following equation:
Gain Block
The LTC3520 contains a gain block (pins A
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
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
push-pull output that can both sink and source current.
The uncommitted amplifi er is internally powered by the
higher of either the SV
the maximum voltage on the A
supply voltage or the buck-boost output voltage, which-
ever is larger.
14
OPERATION
V
LBO
BAT
f kHz
(
OUT
)
Figure 2. Gain Block Used as a Comparator
=
pin is not an open-drain output. Rather, it is a
2.49M
806k
R k
54 000
T
,
(
A
A
OUT
IN
Ω
LTC3520
)
T
IN
pin and ground according to the
or V
PGOOD
OUT1
V
OUT
OUT
voltages. This restricts
pin to either the input
330pF
750k
402k
IN
and A
A
A
OUT
IN
LTC3520
OUT
3520 F02
OUT
T
)
,
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.
200mA
V
3.3V
2.5V
OUT
Figure 3. Gain Block Confi gured as an LDO
4.7µF
169k
76.8k
33pF
A
A
OUT
IN
LTC3520
3520 F05
3520f