SM74203 Datasheet, PDF(15/24 Page) Texas Instruments – SM74203 60V Low Side Controller for Boost and SEPIC

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

SM74203 Datasheet, PDF (15/24 Pages) Texas Instruments – SM74203 60V Low Side Controller for Boost and SEPIC

◁

The next highest standard 20% capacitor value is 6.8 ÂµF, but

because the actual input source impedance and resistance

are not known, two 4.7 ÂµF capacitors will be used. In general,

doubling the calculated value of input capacitance provides a

good safety margin. The final calculation is for the RMS cur-

rent. For boost converters operating in CCM this can be

estimated as:

IRMS = 0.29 x ÎiL(MAX)

From the inductor section, maximum inductor ripple current is

0.58A, hence the input capacitor(s) must be rated to handle

0.29 x 0.58 = 170 mARMS.

The input capacitors can be ceramic, tantalum, aluminum, or

almost any type, however the low capacitance requirement

makes ceramic capacitors particularly attractive. As with the

output capacitors, the minimum quality dielectric used should

X5R, with X7R or better preferred. The voltage rating for input

capacitors need not be as conservative as the output capac-

itors, as the need for capacitance decreases as input voltage

increases. For this example, the capacitor selected will be 4.7

ÂµF Â±20%, rated to 50V, in the 1812 case size. The RMS cur-

rent rating of these capacitors is over 2A each, more than

enough for this application.

CURRENT SENSE FILTER

Parasitic circuit capacitance, inductance and gate drive cur-

rent create a spike in the current sense voltage at the point

where Q1 turns on. In order to prevent this spike from termi-

nating the on-time prematurely, every circuit should have a

low-pass filter that consists of CCS and RS1, shown in Figure

1. The time constant of this filter should be long enough to

reduce the parasitic spike without significantly affecting the

shape of the actual current sense voltage. The recommended

range for RS1 is between 10â¦ and 500â¦, and the recom-

mended range for CCS is between 100 pF and 2.2 nF. For this

example, the values of RS1 and CCS will be 100â¦ and 1 nF,

respectively.

RSNS, RS2 AND CURRENT LIMIT

The current sensing resistor RSNS is used for steady state

regulation of the inductor current and to sense over-current

conditions. The slope compensation resistor is used to ensure

control loop stability, and both resistors affect the current limit

threshold. The RSNS value selected must be low enough to

keep the power dissipation to a minimum, yet high enough to

provide good signal-to-noise ratio for the current sensing cir-

cuitry. RSNS, and RS2 should be set so that the current limit

comparator, with a threshold of 0.5V, trips before the sensed

current exceeds the peak current rating of the inductor, with-

out limiting the output power in steady state.

For this example the peak current, at VIN(MIN), is 2.5A, while

the inductor itself is rated to 3.2A. The threshold for current

limit, ILIM, is set slightly between these two values to account

for tolerance of the circuit components, at a level of 3.0A. The

required resistor calculation must take into account both the

switch current through RSNS and the compensation ramp cur-

rent flowing through the internal 2 kâ¦, RS1 and RS2 resistors.

RSNS should be selected first because it is a power resistor

with more limited selection. The following equation should be

evaluated at VIN(MIN), when duty cycle is highest:

L in ÂµH, fSW in MHz

The closest 5% value is 100 mâ¦. Power dissipation in RSNS

can be estimated by calculating the average current. The

worst-case average current through RSNS occurs at minimum

input voltage/maximum duty cycle and can be calculated as:

PCS = [(0.5 / 0.22)2 x 0.1] x 0.78 = 0.4W

For this example a 0.1â¦ Â±1%, thick-film chip resistor in a 1210

case size rated to 0.5W will be used.

With RSNS selected, RS2 can be determined using the follow-

ing expression:

The closest 1% tolerance value is 3.57 kâ¦.

CONTROL LOOP COMPENSATION

The SM74203 uses peak current-mode PWM control to cor-

rect changes in output voltage due to line and load transients.

Peak current-mode provides inherent cycle-by-cycle current

limiting, improved line transient response, and easier control

loop compensation.

The control loop is comprised of two parts. The first is the

power stage, which consists of the pulse width modulator,

output filter, and the load. The second part is the error ampli-

fier, which is an op-amp configured as an inverting amplifier.

Figure 7 shows the regulator control loop components.

www.national.com

▷