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LTC3733/LTC3733-1

3733f

series of surface-mount tantalums or the Panasonic SP

series of surface mount special polymer capacitors avail-

able in case heights ranging from 2mm to 4mm. Other

capacitor types include Sanyo POS-CAP, Sanyo OS-CON,

Nichicon PL series and Sprague 595D series. Consult the

manufacturer for other specific recommendations.

RSENSE Selection for Output Current

Once the frequency and inductor have been chosen,

RSENSE1, RSENSE2, RSENSE3 are determined based on the

required peak inductor current. The current comparator

has a maximum threshold of 75mV/RSENSE and an input

common mode range of SGND to (1.1) • VCC. The current

comparator threshold sets the peak inductor current,

yielding a maximum average output current IMAX equal to

the peak value less half the peak-to-peak ripple current,

∆IL.

Allowing a margin for variations in the IC and external

component values yields:

SENSE

MAX

The IC works well with values of RSENSE from 0.001Ω to

0.02

Ω.

VCC Decoupling

The VCC pin supples power not only the internal circuits of

the controller but also the top and bottom gate drivers and

therefore must be bypassed very carefully to ground with

a ceramic capacitor, type X7R or X5R (depending upon

the operating temperature environment) of

at least 1

µF

immediately next to the IC and preferably an additional

µF placed very close to the IC due to the extremely high

instantaneous currents involved. The total capacitance,

taking into account the voltage coefficient of ceramic

capacitors, should be 100 times as large as the total

combined gate charge capacitance of ALL of the MOSFETs

being driven. Good bypassing close to the IC is necessary

to supply the high transient currents required by the

MOSFET gate drivers while keeping the 5V supply quiet

enough so as not to disturb the very small-signal high

bandwidth of the current comparators.

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors, CB, connected to the BOOST

pins, supply the gate drive voltages for the topside

MOSFETs. Capacitor CB in the Functional Diagram is

charged though diode DB from VCC when the SW pin is

low. When one of the topside MOSFETs turns on, the

driver places the CB voltage across the gate-source of the

desired MOSFET. This enhances the MOSFET and turns on

the topside switch. The switch node voltage, SW, rises to

VIN and the BOOST pin follows. With the topside MOSFET

on, the boost voltage is above the input supply (VBOOST =

VCC + VIN). The value of the boost capacitor CB needs to be

30 to 100 times that of the total input capacitance of the

topside MOSFET(s). The reverse breakdown of DB must be

greater than VIN(MAX).

Differential Amplifier

The IC has a true remote voltage sense capability. The

sensing connections should be returned from the load,

back to the differential amplifier’s inputs through a com-

mon, tightly coupled pair of PC traces. The differential

amplifier rejects common mode signals capacitively or

inductively radiated into the feedback PC traces as well as

ground loop disturbances. The differential amplifier out-

put signal is divided down through the VID DAC and is

compared with the internal, precision 0.6V voltage refer-

ence by the error amplifier.

The amplifier has a 0 to VCC common mode input range

and an output swing range of 0 to VCC – 1.2V. The output

uses an NPN emitter follower with 80k

Ω feedback resis-

tance. A DC resistive load to ground is required in order to

sink more current.

APPLICATIO S I FOR ATIO

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