TPS40020

TPS40021

SLUS535D − MARCH 2003 − REVISED JULY 2007

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26

REFERENCE DESIGN

MOSFETs and Diodes

For a 1.5-V output voltage, the lower the RDS(on) of the MOSFET, the higher the efficiency. Due to the high

current and high conduction loss, the MOSFET should have very low conduction resistance (RDS(on)) and

thermal resistance. Si7858DP is chosen for its low RDS(on) (between 3 mΩ and 4 mΩ) and Power-Pak package.

Current Limiting

Resistor R3 sets the short current limit threshold. The RDS(on) of the upper MOSFET is used as a current sensor.

The current limit, IOUT(CL) is initialized at 30% above the maximum output current, IOUT(max), which is 28 A. Then

R3 can be calculated in equation (11) and yields a value of 1.4 k

Ω. An R3 of 1.43 kΩ is selected.

I

LIM +

19

V

FB

R4

+ 19

0.69 V

118 k

W +

111.1 (

mA)

R3

+

K

R

DS(on)

I

OUT(CL)

I

LIM

+ 1.5

0.004

28 A

I

LIM

+ 1.4 (kW)

where

D RDS(on) is the on-resistor of Q1 (4 mΩ)

D Temperature coefficient, K=1.5

D VFB = 0.69 V

D R4=118 kΩ

Voltage Sense Regulator

R1 and R2 operate as the output voltage divider. The error amplifier reference voltage (VFB) is 0.69 V. The

relationship between the output voltage and divider is described in equation (8). Using a 10-k

Ω resistor for R2

and 1.5-V output regulation, R1 is calculated as 8.52 k

Ω, 8.66 kΩ is selected for R1.

V

FB

R1

+

V

OUT

R1

) R2

³ 0.69 V

R1

+

1.5 V

R1

) 10 kW

³ R1 + 8.52 kW

Transient Comparator

The output voltage transient comparators provide a quick response, first strike, approach to output voltage

transients. The output voltage is sensed through a resistor divider at the OSNS pin, using R5 and R6 shown

in Figure 27. If an overvoltage condition is detected, the HDRV gate drive is shut off and the LDRV gate drive

is turned on until the output is returned to regulation. Similarly, if an output undervoltage condition is sensed,

the HDRV gate drive goes to 95% duty cycle to pump the output back up quickly. The voltage divider should

be exactly the same as resistors R1 and R2 discussed previously. Resistor R5=8.66 k

Ω and R6=10 kΩ in this

evaluation module.

(14)

(15)

(16)

Not Recommended for New Designs