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LTC3704

sn3704 3704fs

Applications Circuits

A simple positive-to-negative application circuit for the

LTC3704 is shown in Figure 1. The basic operation of this

circuit is shown in Figure 9. During the on-time the

inductor currents flow through the switch, and during the

off-time these currents flow through the output diode. The

use of inductors in series with both the input and output

results in continuous currents in these capacitors, result-

ing in low input and output noise. Discontinuous currents

flow in the switch, the coupling capacitor, and the diode.

Peak and Average Input and Switch Currents

The control loop in the LTC3704 is measuring the peak

switch current (either by using the RDS(ON) of the power

MOSFET or by using a sense resistor in the MOSFET

source), so the output current needs to be reflected back

to the switch in order to dimension the power MOSFET and

inductors properly. Based on the fact that, ideally, the

input power is equal to the output power, the maximum

average input current is:

IN MAX

O MAX

MAX

()

–•

–

where IO(MAX) is a negative number. The peak input

current is:

IN PEAK

O MAX

MAX

()

(

)

••

–

=− +









In a positive-to-negative converter, however, the switch

current is equal to IIN + IO, so the maximum average switch

current is:

SW MAX

O MAX

MAX

()

() •

=−

−

and the peak switch current is:

SW PEAK

O MAX

MAX

()

(

)

••

–

=− +









The maximum duty cycle, DMAX, should be calculated at

minimum VIN.

Ripple Current

∆IL and the ‘χ’ Factor

The constant ‘χ’ in the equation above represents the

percentage peak-to-peak total ripple current in the induc-

tor, relative to its maximum value. For example, if 30%

ripple current is chosen, then χ = 0.30, and the peak

current is 15% greater than the average.

For a current mode converter operating in CCM, slope

compensation must be added for duty cycles above 50%

in order to avoid subharmonic oscillation. For the LTC3704,

this ramp compensation is internal. Having an internally

fixed ramp compensation waveform, however, does place

some constraints on the value of the inductor and the

operating frequency. If too large an inductor is used, the

resulting current ramp (

∆IL) will be small relative to the

APPLICATIO S I FOR ATIO

Figure 9. Positive-to-Negative Converter Operation

Duty Cycle Considerations

For the positive-to-negative converter shown in Figure 1,

the duty cycle of the main switch in CCM is:

OIN

–

where VO is a negative number. The maximum output

voltage for this converter (in CCM) is:

O MAX

IN MIN

MAX

()

(

) •

–

The maximum duty cycle capability of the LTC3704 is

typically 92%.

+–

VOUT

VIN

a) Current Flow During The Switch On-Time

+–

OFF

VOUT

VIN

b) Current Flow During The Switch Off-Time

3704 F09

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