LT3570
16
3570fb
Oscillator
The free-running frequency is set through a resistor from
the R
T
pin to ground. The oscillator frequency vs R
T
can
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.
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
1糉 up to 100糉. 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 ef ciently. The power
Dissipated in the external NPN is equal to:
P
DISS
= (V
COL
V
OUT3
) " I
LOAD
where V
COL
is the collector voltage of the NPN. The maxi-
mum output voltage is limited to:
V
IN3
1.4V and V
COL
0.2V or 8V
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癈. When
the die temperature has dropped below this value the part
APPLICATIONS INFORMATION
Figure 4. Frequency vs R
T
Resistance
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
10約 allowing the inductor current to decrease. The 10約
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.
RESISTANCE (k?
5
1250
1750
45
3570 F04
750
250
15
25
35
10
20
30
40
2250
1000
1500
500
2000
FREQUENCY (kHz)
500
0
5
10
15
20
25
750 1000 1250 1500
3570 F05
1750 2000
Figure 5. Minimum Duty Cycle vs Frequency
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