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LT1941

1941f

APPLICATIO S I FOR ATIO

Boost Pin Considerations

The capacitor and diode tied to the BOOST pin generate a

voltage that is higher than the input voltage. In most

cases, a 0.18

µF capacitor and fast switching diode (such

as the CMDSH-3 or MMSD914LT1) will work well. Fig-

ure 3 shows two ways to arrange the boost circuit. The

BOOST pin must be more than 2.5V above the SW pin for

full efficiency. For outputs of 3.3V and higher, the stan-

dard circuit (Figure 3a) is best. For outputs between 2.8V

and 3.3V, use a small Schottky diode (such as the

BAT-54). For lower output voltages, the boost diode can

be tied to the input (Figure 3b). The circuit in Figure 3a is

more efficient because the boost pin current comes from

a lower voltage source. Finally, as shown in Figure 3c, the

anode of the boost diode can be tied to another source that

is at least 3V. For example, if you are generating 3.3V and

1.8V and the 3.3V is on whenever the 1.8V is on, the 1.8V

boost diode can be connected to the 3.3V output. In any

case, be sure that the maximum voltage at the BOOST pin

is less than 35V and the voltage difference between the

BOOST and SW pins is less than 25V.

The boost circuit can also run directly from a DC voltage

that is higher than the input voltage by more than 2.5V +

VF, as in Figure 3d. The diode prevents damage to the

LT1941 in case VIN2 is held low while VIN is present. The

circuit saves several components (both BOOST pins can

be tied to D2). However, efficiency may be lower and

dissipation in the LT1941 may be higher. Also, if VIN2 is

absent the LT1941 will still attempt to regulate the output,

but will do so with low efficiency and high dissipation

because the switch will not be able to saturate, dropping

1.5 to 2V in conduction.

The minimum operating voltage of an LT1941 application

is limited by the undervoltage lockout (3.5V) and by the

maximum duty cycle. The boost circuit also limits the

minimum input voltage for proper start-up. If the input

voltage ramps slowly, or the LT1941 turns on when the

output is already in regulation, the boost capacitor may not

be fully charged. Because the boost capacitor charges

with the energy stored in the inductor, the circuit will rely

on some minimum load current to get the boost circuit

running properly. This minimum load will depend on input

and output voltages, and on the arrangement of the boost

circuit. The minimum load current generally goes to zero

once the circuit has started. Even without an output load

current, in many cases the discharged output capacitor

will present a load to the switcher that will allow it to start.

Figure 3. Generating the Boost Voltage

VIN

BOOST

GND

VIN

LT1941

(3a)

VOUT

VBOOST – VSW ≅ VOUT

MAX VBOOST ≅ VIN + VOUT

VIN

BOOST

GND

VIN

LT1941

(3b)

VOUT

VBOOST – VSW ≅ VIN

MAX VBOOST ≅ 2VIN

VIN

BOOST

GND

VIN

LT1941

(3d)

1941 F03

VOUT

MAX VBOOST – VSW ≅ VIN2

MAX VBOOST ≅ VIN2

MINIMUM VALUE FOR VIN2 = VIN + 3V

VIN2

>VIN + 3V

VIN

BOOST

GND

VIN

LT1941

(3c)

VOUT

VBOOST – VSW ≅ VIN2

MAX VBOOST ≅ VIN2 + VIN

MINIMUM VALUE FOR VIN2 = 3V

VIN2 > 3V

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