LTC1876 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC1876

High Efficiency, 2-Phase, Dual Synchronous Step-Down Switching Controller and Step-Up Regulator

Linear Technology 

LTC1876

APPLICATIO S I FOR ATIO

Choosing 1% resistors; R1 = 25.5k and R2 = 32.4k yields

an output voltage of 1.816V.

The power dissipation on the top side MOSFET can be

easily estimated. Choosing a Siliconix Si4412DY results

in; RDS(ON) = 0.042Ω, CRSS = 100pF. At maximum input

voltage with T(estimated) = 50°C:

( ) [ ] PMAIN

=

1.8V

22V

2

5

1+ (0.005)(50°C – 25°C)

( ) ( ) ( )( )( ) 2

0.042Ω + 1.7 22V 5A 100pF 300kHz

= 220mW

A short-circuit to ground will result in a folded back

current of:

ISC

=

25mV

0.01Ω

+

1 200ns(22V)

2  3.3µH 

=

3.2A

with a typical value of RDS(ON) and δ = (0.005/°C)(20)

= 0.1. The resulting power dissipated in the bottom

MOSFET is:

( ) ( )( ) PSYNC

=

22V – 1.8V

22V

3.2A

2

1.1

0.042Ω

= 434mW

which is less than under full-load conditions.

CIN is chosen for an RMS current rating of at least 3A at

temperature assuming only this channel is on. COUT is

chosen with an ESR of 0.02Ω for low output ripple. The

output ripple in continuous mode will be highest at the

maximum input voltage. The output voltage ripple due to

ESR is approximately:

VORIPPLE = RESR(∆IL) = 0.02Ω(1.67A) = 33mVP–P

Design Example for Auxiliary Regulator

Assume the requirements are VIN = 5V, VOUT = 12V and

IOUTMAX = 300mA. The duty cycle is given by:

Duty Cycle = 1– VIN = 0.58

VOUT

Since the required output current is 300mA, the ripple

current of the inductor is calculated to be 0.57A.

Hence the required inductor is:

L = (VIN •Duty Cycle)

(f • ∆IL)

With the boost regulator operating at 1.2MHz,

L = 4.24µH

A 10µH inductor is selected for the circuit for lower ripple

inductor current. Since the output current is only 300mA,

a 0.5A MBR0520 Schottky is selected. The completed

circuit along with its efficiency curve is shown in Figure 13

and Figure 14 respectively.

VIN3

5V

CIN3

2.2µF

L3

10µH

SHDN

AUXVIN AUXSW

LTC1876

AUXSD AUXVFB

SGND

D1

R8

113k

R7

13.3k

C3*

10pF

VOUT3

12V

300mA

+

COUT3

4.7µF

C1: TAIYO YUDEN X5R LMK212BJ225MG

C2: TAIYO YUDEN X5R EMK316BJ475ML

D1: ON SEMICONDUCTOR MBR0520

L1: SUMIDA CR43-100

*OPTIONAL

1876 F13

Figure 13. Design Example Schematic

90

VIN = 5V

85

VIN = 3.3V

80

75

70

65

60

55

50

0

100

200

300

400

LOAD CURRENT (mA)

1876 F14

Figure 14. Efficiency Curve for Design Example

1876fa

29

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