LTC3548 datasheet(18/30 Pages) LINER

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LT3510

3510fe

input and output voltages, and on the arrangement of the

boost circuit. The Typical Performance Characteristics

section shows plots of the minimum load current to start

and to run as a function of input voltage for 3.3V and 5V

outputs. In many cases the discharged output capacitor

will present a load to the switcher which will allow it to

start. The plots show the worst-case situation where VIN is

ramping very slowly. Use a Schottky diode for the lowest

start-up voltage.

Frequency Compensation

The LT3510 uses current mode control to regulate the

output. This simpliﬁes loop compensation. In particular, the

LT3510 does not require the ESR of the output capacitor

for stability so you are free to use ceramic capacitors to

achieve low output ripple and small circuit size.

Frequency compensation is provided by the components

tied to the VC pin. Generally a capacitor and a resistor in

series to ground determine loop gain. In addition, there

is a lower value capacitor in parallel. This capacitor is not

part of the loop compensation but is used to ﬁlter noise

at the switching frequency.

Loop compensation determines the stability and transient

performance. Designing the compensation network is a bit

complicated and the best values depend on the application

and in particular the type of output capacitor. A practical

approach is to start with one of the circuits in this data

sheet that is similar to your application and tune the com-

pensation network to optimize the performance. Stability

should then be checked across all operating conditions,

including load current, input voltage and temperature.

The LT1375 data sheet contains a more thorough discus-

sion of loop compensation and describes how to test the

stability using a transient load.

Figure 6 shows an equivalent circuit for the LT3510 control

loop. The error amp is a transconductance ampliﬁer with

ﬁnite output impedance. The power section, consisting of

the modulator, power switch and inductor, is modeled as

a transconductance ampliﬁer generating an output cur-

rent proportional to the voltage at the VC pin. Note that

Figure 5. BST Pin Considerations

VIN

VX > VIN + 3V

BST

GND

LT3510

IND

VOUT

VBST – VSW = VX

VBST(MAX) = VX

VX(MIN) = VIN + 3V

VOUT < 3V

3510 F05

VIN

VX = LOWEST VIN

OR VOUT > 3V

BST

GND

LT3510

IND

VOUT

VBST – VSW = VX

VBST(MAX) = VIN + VX

VX(MIN) = 3V

VOUT < 3V

VIN

BST

GND

(5d)

(5c)

(5b)

LT3510

IND

VOUT

VBST – VSW = VIN

VBST(MAX) = 2 • VIN

VOUT < 3V

VIN

BST

GND

(5a)

LT3510

IND

VOUT

VBST – VSW = VOUT

VBST(MAX) = VIN + VOUT

VOUT

APPLICATIONS INFORMATION

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