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LT3437

3437fc

These requirements result in the use of a Schottky type

diode. DC switching losses are minimized due to its low

forward voltage drop, and AC behavior is benign due to its

lack of a significant reverse recovery time. Schottky diodes

are generally available with reverse voltage ratings of 60V,

and even 100V, and are price competitive with other types.

The use of so-called “ultrafast” recovery diodes is gener-

ally not recommended. When operating in continuous

mode, the reverse recovery time exhibited by “ultrafast”

diodes will result in a slingshot type effect. The power

internal switch will ramp up VIN current into the diode in an

attempt to get it to recover. When the diode has finally

turned off, some tens of nanoseconds later, the VSW node

voltage ramps up at an extremely high dV/dt, perhaps 5V

to even 10V/ns! With real world lead inductances, the VSW

node can easily overshoot the VIN rail. This can result in

poor RFI behavior, and if the overshoot is severe enough,

damage the IC itself.

BOOST PIN

For most applications, the boost components are a 0.1

µF

capacitor and a BAS21 diode. The anode is typically

connected to the regulated output voltage, to generate a

voltage approximately VOUT above VIN to drive the output

stage (Figure 6a). However, the output stage discharges

the boost capacitor during the on time of the switch. The

output driver requires at least 2.5V of headroom through-

out this period to keep the switch fully saturated. If the

output voltage is less than 3.3V, it is recommended that an

alternate boost supply is used. The boost diode can be

connected to the input (Figure 6b), but care must be taken

to prevent the boost voltage (VBOOST = VIN • 2) from

exceeding the BOOST pin absolute maximum rating. The

additional voltage across the switch driver also increases

power loss and reduces efficiency. If available, an inde-

pendent supply can be used to generate the required

BOOST voltage (Figure 6c). Tying BOOST to VIN or an

independent supply may reduce efficiency, but it will

reduce the minimum VIN required to start-up with light

loads. If the generated BOOST voltage dissipates too

much power at maximum load, the BOOST voltage the

LT3437 sees can be reduced by placing a Zener diode in

series with the BOOST diode (Figure 6a option).

A 0.1

µF boost capacitor is recommended for most appli-

cations. Almost any type of film or ceramic capacitor is

suitable, but the ESR should be <1

Ω to ensure it can be

fully recharged during the off time of the switch. The

capacitor value is derived from worst-case conditions of

4700ns on time, 11mA boost current and 0.7V discharge

ripple. The boost capacitor value could be reduced under

less demanding conditions, but this will not improve

circuit operation or efficiency. Under low input voltage and

low load conditions, a higher value capacitor will reduce

discharge ripple and improve start-up operation.

BOOST

LT3437

VBOOST – VSW = VOUT

VBOOST(MAX) = VIN + VOUT

VIN

VOUT

OPTIONAL

(6a)

VIN

GND

BOOST

LT3437

VBOOST – VSW = VDC

VBOOST(MAX) = VDC + VIN

VIN

VDC

DSS

3437 F06

VOUT

(6c)

VIN

GND

BOOST

LT3437

VBOOST – VSW = VIN

VBOOST(MAX) = 2VIN

VIN

VOUT

(6b)

VIN

GND

Figure 6. BOOST Pin Configurations

APPLICATIO S I FOR ATIO

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