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LT3470A

3470afa

APPLICATIONS INFORMATION

(Figure 2b). The circuit in Figure 2a is more efﬁcient

because the BOOST pin current and BIAS pin quiescent

current comes from a lower voltage source. You must also

be sure that the maximum voltage ratings of the BOOST

and BIAS pins are not exceeded.

The LT3470A monitors the boost capacitor for sufﬁcient

voltage such that the switch is allowed to fully saturate.

When boost voltage falls below adequate levels (1.8V

typical) the switch will operate with about 1V of drop, and

an internal current source will begin to pull 50mA (typi-

cal) from the BIAS pin which is typically connected to the

output. This current forces the LT3470A to switch more

often and with more inductor current, which recharges

the boost capacitor. When the boost capacitor voltage is

above 1.8V (typical) the current source turns off, and the

part may enter BurstMode. This cycle will repeat anytime

there is an undervoltage condition on the boost capaci-

tor. See Figure 3 for minimum input voltage for outputs

of 3.3V and 5V.

Shorted Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively at the top switch current limit maximum of 525mA,

an LT3470A buck regulator will tolerate a shorted output

even if VIN = 40V. There is another situation to consider

in systems where the output will be held high when the

input to the LT3470A is absent. This may occur in battery

charging applications or in battery backup systems where

a battery or some other supply is diode OR-ed with the

LT3470A’s output. If the VIN pin is allowed to ﬂoat and the

SHDN pin is held high (either by a logic signal or because

it is tied to VIN), then the LT3470A’s internal circuitry will

pull its quiescent current through its SW pin. This is ﬁne

if your system can tolerate a few mA in this state. If you

ground the SHDN pin, the SW pin current will drop to es-

sentially zero. However, if the VIN pin is grounded while

the output is held high, then parasitic diodes inside the

LT3470A can pull large currents from the output through

the SW pin and the VIN pin. Figure 4 shows a circuit that

will run only when the input voltage is present and that

protects against a shorted or reversed input.

Figure 3. The Minimum Input Voltage Depends on Output

Voltage, Load Current and Boost Circuit

Minimum Input Voltage, VOUT = 3.3V

Minimum Input Voltage, VOUT = 5V

Figure 4. Diode D1 Prevents a Shorted Input from Discharging

a Backup Battery Tied to the Output; It Also Protects the Circuit

from a Reversed Input. The LT3470A Runs Only When the Input is

Present Hot-Plugging Safely

VIN

BOOST

LT3470A

SHDN

3470a F04

VIN

100k

VOUT

BACKUP

BIAS

GND

LOAD CURRENT (mA)

3.0

3.5

4.0

4.5

5.0

5.5

6.0

100

150

200

250

3470a F03a

TA = 25°C

VIN TO RUN/START

LOAD CURRENT (mA)

250

3470a F03b

100

150

200

TA = 25°C

VIN TO RUN/START

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