lt3570 Linear Technology Corporation, lt3570 Datasheet - Page 16

• Manufacturer Part Number: lt3570
• Description: 1.5a Buck Converter, 1.5a Boost Converter And Ldo Controller
• Manufacturer: Linear Technology Corporation

LT3570
APPLICATIONS INFORMATION
Oscillator
The free-running frequency is set through a resistor from
the R
be seen in Figure 4. The oscillator can be synchronized
with an external clock applied to the SYNC pin. When
synchronizing the oscillator, the free running frequency
must be set approximately 10% lower than the desired
synchronized frequency.
Buck Regulator Minimum On-Time
As the input voltage is increased, the LT3570 is required
to turn on for shorter periods of time. Delays associated
with turning off the power switch determine the minimum
on-time that can be achieved and limit the minimum duty
cycle. Figure 5 shows the minimum duty cycle versus
frequency for the LT3570. When the required on-time has
decreased below the minimum on-time of the LT3570 the
inductor current will increase, exceeding the current limit.
If the current through the inductor exceeds the current limit
of the LT3570, the switch is prevented from turning on for
10μs allowing the inductor current to decrease. The 10μs
off-time limits the average current that can be delivered to
the load. To return to normal switching frequency either
the input voltage or load current must decrease.
16
T
pin to ground. The oscillator frequency vs R
2250
2000
1750
1500
1250
1000
750
500
250
Figure 4. Frequency vs R
5
10
15
RESISTANCE (kΩ)
20
25
30
T
35
Resistance
40
3570 F04
45
T
can
LDO Regulator
The LT3570 LDO regulator is capable of delivering up to
10mA of base drive for an external NPN transistor. For
stable operation the total output capacitance can be from
1μF up to 100μF . The regulator has its own independent
supply voltage which allows for the base of the NPN to be
driven from a higher voltage than its collector. This allows
for the NPN regulator to run more effi ciently. The power
Dissipated in the external NPN is equal to:
where V
mum output voltage is limited to:
The short-circuit protection of the NPN regulator is set by
the max output current of the NPN_DRV pin multiplied by
the beta of the NPN.
Thermal Shutdown
An internal temperature monitor will turn off the internal
circuitry and prevent the switches from turning on when
the die temperature reaches approximately 160°C. When
the die temperature has dropped below this value the part
P
V
DISS
IN3
– 1.4V and V
COL
= (V
Figure 5. Minimum Duty Cycle vs Frequency
25
20
15
10
is the collector voltage of the NPN. The maxi-
5
0
500
COL
– V
750
OUT3
COL
1000
FREQUENCY (kHz)
– 0.2V or 8V
) • I
1250
LOAD
1500
1750 2000
3570 F05
3570fa