LM25017 datasheet(13/19 Pages) TI

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RUV2

VIN (UVLO,rising) = 1.225V x

RUV1

+ 1

(

)

VIN(HYS) = IHYS x RUV2

IOUT(MAX)

CIN

8 x gSW x ûVIN

25 mV

ûIL(MIN)

VOUT

VREF

LM25017

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SNVS951 – DECEMBER 2012

where

ΔVripple is the voltage ripple across the capacitor. Substituting ΔVripple = 10mV gives COUT = 12.64μF. A

μF standard value is selected. An X5R or X7R type capacitor with a voltage rating 16V or higher should be

selected.

Series Ripple Resistor RC:

The series resistor should be selected to produce sufficient ripple at the feedback node. The ripple produced by

RC is proportional to the inductor current ripple, and therefore RC should be chosen for minimum inductor current

ripple which occurs at minimum input voltage. The RC is calculated by the equation:

(12)

This gives an RC of greater than or equal to 5.15Ω. Selecting RC = 5.23Ω results in ~1V of maximum output

voltage ripple. For applications requiring lower output voltage ripple, Type II or Type III ripple injection circuits

should be used as described in the section “Ripple Configuration”.

VCC and Bootstrap Capacitor:

The VCC capacitor provides charge to bootstrap capacitor as well as internal circuitry and low side gate driver.

The Bootstrap capacitor provides charge to high side gate driver. A good value for CVCC is 1μF. A good value for

CBST is 0.01μF.

Input Capacitor:

Input capacitor should be large enough to limit the input voltage ripple:

(13)

choosing a

ΔVIN = 0.5V gives a minimum CIN = 1.24μF. A standard value of 2.2μF is selected. The input

capacitor should be rated for the maximum input voltage under all conditions. A 50V, X7R dielectric should be

selected for this design.

Input capacitor should be placed directly across VIN and RTN (pin 2 and 1) of the IC. If it is not possible to place

all of the input capacitor close to the IC, a 0.47

μF capacitor should be placed near the IC to provide a bypass

path for the high frequency component of the switching current. This helps limit the switching noise.

UVLO Resistors:

The UVLO resistors RFB1 and RFB2 set the UVLO threshold and hysteresis according to the following relationship:

(14)

and

(15)

where IHYS = 20μA. Setting UVLO hysteresis of 2.5V and UVLO rising threshold of 12V results in RUV1 = 14.53kΩ

and RUV2 = 125kΩ. Selecting standard value of RUV1 = 14kΩ and RUV2 = 125kΩ results in UVLO thresholds and

hysteresis of 12.4V and 2.5V respectively.

APPLICATION CIRCUIT: 12V TO 48V INPUT AND 10V, 500mA OUTPUT BUCK CONVERTER

The application schematic of a buck supply is shown in Figure 16 below. For output voltage (VOUT) above the

maximum regulation threshold of VCC (8.55V, see electrical characteristics), the VCC pin can be connected to

VOUT through a diode (D2), as shown below, for higher efficiency and lower power dissipation in the IC.

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