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LTC1778/LTC1778-1

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APPLICATIO S I FOR ATIO

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 ∆ILOAD (ESR), where ESR is the effective series

resistance of COUT. ∆ILOAD also begins to charge or

discharge 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 prob-

lem. The ITH pin external components shown in Figure 9

will provide adequate compensation for most applica-

tions. For a detailed explanation of switching control loop

theory see Application Note 76.

Design Example

As a design example, take a supply with the following

specifications: VIN = 7V to 28V (15V nominal), VOUT = 2.5V

±5%, IOUT(MAX) = 10A, f = 250kHz. First, calculate the

timing resistor with VON = VOUT:

kHz

ON = ()(

)(

)

Ω

0 7

250

142

and choose the inductor for about 40% ripple current at

the maximum VIN:

kHz

= ()( )( ) −

⎛

⎝

⎜

⎞

⎠

⎟ =µ

250

0 4 10

Selecting a standard value of 1.8µH results in a maximum

ripple current of:

∆ = () µ

()

⎛

⎝

⎜

⎞

⎠

⎟ =

kHz

250

1 8

–

Next, choose the synchronous MOSFET switch. Choosing

a Si4874 (RDS(ON) = 0.0083Ω (NOM) 0.010Ω (MAX),

θJA = 40°C/W) yields a nominal sense voltage of:

VSNS(NOM) = (10A)(1.3)(0.0083Ω) = 108mV

Tying VRNG to 1.1V will set the current sense voltage range

for a nominal value of 110mV with current limit occurring

at 146mV. To check if the current limit is acceptable,

assume a junction temperature of about 80°C above a

70°C ambient with ρ150°C = 1.5:

LIMIT ≥ ()

Ω

()

() =

146

15 0010

and double check the assumed TJ in the MOSFET:

BOT =

() ( )

Ω

() =

2 5

15 0 010

197

–.

TJ = 70°C + (1.97W)(40°C/W) = 149°C

Because the top MOSFET is on for such a short time, an

Si4884 RDS(ON)(MAX) = 0.0165Ω, CRSS = 100pF, θJA =

40°C/W will be sufficient. Checking its power dissipation

at current limit with ρ100°C = 1.4:

VApF

kHz

TOP =

() ( )

Ω

()+

()( ) ( )(

)(

)

=+=

1 4 0 0165

1 7 28

100

250

030

0 40

TJ = 70°C + (0.7W)(40°C/W) = 98°C

The junction temperatures will be significantly less at

nominal current, but this analysis shows that careful

attention to heat sinking will be necessary in this circuit.

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