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LTC3823

3823fd

the load current and the type of MOSFET used, users need

to be careful to optimize the dead time for their particular

applications. Figure 11 shows the relation between the TG

Low BG High dead time by varying the Z0 voltages. For

an application using LTC3823 with load current of 5A and

IR7811W MOSFETs, the dead time could be optimized. To

make sure that there is no shoot-through under all condi-

tions, a dead time of 70ns is selected. This corresponds to

a DC voltage about 2.4V on Z0 pin. This voltage can easily

be generated with a resistor divider off INTVCC.

APPLICATIONS INFORMATION

2. Transition loss. This loss arises from the brief amount

of time the top MOSFET spends in the saturated region

during switch node transitions. It depends upon the

input voltage, load current, driver strength and MOSFET

capacitance, among other factors. The loss is signiﬁ-

cant at input voltages above 20V and can be estimated

from:

Transition Loss

≅ (1.7A–1) VIN2 IOUT CRSS f

3. INTVCC current. This is the sum of the MOSFET driver

and control currents.

4. CIN loss. The input capacitor has the difﬁcult job of

ﬁltering the large RMS input current to the regulator. It

must have a very low ESR to minimize the AC I2R loss

and sufﬁcient capacitance to prevent the RMS current

from causing additional upstream losses in fuses or

batteries.

Other losses, including COUT ESR loss, Schottky diode D1

conduction loss during dead time and inductor core loss

generally account for less than 2% additional loss.

When making adjustments to improve efﬁciency, the input

current is the best indicator of changes in efﬁciency. If you

make a change and the input current decreases, then the

efﬁciency has increased. If there is no change in input

current, then there is no change in efﬁciency.

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

a load step occurs, VOUT immediately shifts by an amount

equal to ΔILOAD (ESR), where ESR is the effective series

resistance of COUT. ΔILOAD also begins to charge or dis-

charge COUT generating a feedback error signal used by the

regulator to return VOUT to its steady-state value. During

this recovery time, VOUT can be monitored for overshoot

or ringing that would indicate a stability problem. The ITH

pin external components shown in Figure 12 will provide

adequate compensation for most applications. For a

detailed explanation of switching control loop theory see

Application Note 76.

Z0 VOLTAGE (V)

120

160

200

3823 F11

100

140

180

0.5

1.5

3.5

4.5

2.5

IOUT = 2A

FIGURE 12 CIRCUIT

Figure 11. TG Low BG High Dead Time vs Z0 Voltage

Efﬁciency Considerations

The percent efﬁciency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efﬁciency and which change would

produce the most improvement. Although all dissipative

elements in the circuit produce losses, four main sources

account for most of the losses in LTC3823 circuits:

1. DC I2R losses. These arise from the resistances of the

MOSFETs, inductor and PC board traces and cause the

efﬁciency to drop at high output currents. In continuous

mode the average output current ﬂows through L, but is

choppedbetweenthetopandbottomMOSFETs.Ifthetwo

MOSFETs have approximately the same RDS(ON), then

the resistance of one MOSFET can simply be summed

with the resistances of L and the board traces to obtain

the DC I2R loss. For example, if RDS(ON) = 0.01Ω and

RL = 0.005Ω, the loss will range from 15mW to 1.5W

as the output current varies from 1A to 10A.

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