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LTC1267-ADJ

Dual High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

LTC1267

LTC1267-ADJ/LTC1267-ADJ5

UU W U

APPLICATIO S I FOR ATIO

Where δ is the temperature dependency of RDS(ON) and k

is a constant inversely related to the gate drive current.

Both MOSFETs have I2R losses, while the P-channel

equation includes an additional term for transition losses,

which are highest at high input voltages. For VIN < 20V, the

high current efficiency generally improves with larger

MOSFETs, while for VIN > 20V, the transition losses rapidly

increase to the point that the use of a higher RDS(ON) device

with lower CRSS actually provides higher efficiency. The

N-channel MOSFET losses are the greatest at high input

voltage or during a short circuit when the N-channel duty

cycle is nearly 100%.

The term (1 + δ) is generally given for a MOSFET in the

form of a normalized RDS(ON) vs temperature curve, but δ

= 0.007/°C can be used as an approximation for low

voltage MOSFETs. CRSS is usually specified in the MOSFET

electrical characteristics. The constant k = 5 can be used

for the LTC1267 to estimate the relative contributions of

the two terms in the P-channel dissipation equation.

The Schottky diodes D3 and D5 shown in Figure 1 only

conduct during the dead-time between the conduction of

the respective power MOSFETs. The sole purpose of D3

and D5 is to prevent the body diode of the N-channel

MOSFET from turning on and storing charge during the

dead-time, which could cost as much as 1% in efficiency

(although there are no other harmful effects if D3 and D5

are omitted). Therefore, D3 and D5 should be selected for

a forward voltage of less than 0.6V when conducting IMAX.

CIN and COUT Selection

In continuous mode, the source current of the P-channel

MOSFET is a square wave of duty cycle VOUT/ VIN. To

prevent large voltage transients, a low ESR input capaci-

tor sized for the maximum RMS current must be used. The

maximum RMS capacitor current is given by:

CIN

Required

IRMS

≈

IMAX

[VOUT(VIN – VOUT)]1/2

VIN

This formula has a maximum at VIN = 2VOUT where IRMS =

IOUT/2. This simple worst-case condition is commonly

used for design because even significant deviations do not

offer much relief. Note that capacitor manufacturer’s

ripple current ratings are often based on only 2000 hours

of life. This makes it advisable to further derate the

capacitor or to choose a capacitor rated at a higher

temperature than required. Several capacitors may also be

paralleled to meet size or height requirements in the

design. Always consult the manufacturer if there is any

question. An additional 0.1µF ceramic capacitor is also

required on VIN for high frequency decoupling.

The selection of COUT is driven by the required Effective

Series Resistance (ESR). The ESR of COUT must be less

than twice the value of RSENSE for proper operation of the

LTC1267:

COUT Required ESR < 2RSENSE

Optimum efficiency is obtained by making the ESR equal

to RSENSE. As the ESR is increased up to 2RSENSE, the

efficiency degrades by less than 1%. If the ESR is greater

than 2RSENSE, the voltage ripple on the output capacitor

will prematurely trigger Burst Mode operation, resulting in

disruption of continuous mode and an efficiency hit which

can be several percent.

Manufacturers such as Nichicon, United Chemicon, and

Sprague should be considered for high performance ca-

pacitors. In surface mount applications multiple capaci-

tors may have to be paralleled to meet the capacitance,

ESR, or RMS current handling requirements of the appli-

cation. For additional information regarding capacitor

selection, please refer to the LTC1159 data sheet.

At low supply voltages, a minimum capacitance at COUT is

needed to prevent an abnormal low frequency operating

mode (see Figure 4). When COUT is made too small, the

output ripple at low frequencies will be large enough to trip

1000

800

600

L = 50µH

RSENSE = 0.02Ω

L = 25µH

RSENSE = 0.02Ω

400

L = 50µH

RSENSE = 0.05Ω

200

0

0

1

2

3

4

5

(VIN – VOUT) VOLTAGE (V)

LTC1267 • F04

Figure 4. Minimum Suggested COUT

10

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