LT1508CSW datasheet(9/16 Pages) LINER

9 / 16 page

LT1508

(PIN)(RS)(25)(RIAC + 25k)

(VIN2)(RREF)

VAOUT = 2 +

See Figure 2 for RREF.

VAOUT is squared in the multiplier, resulting in excellent

performance over a wide range of output power and input

voltage without the addition of feedforward line frequency

ripple. Care must be taken to avoid feeding switching

frequency noise into the multiplier from the IAC pin. An

internal 25k is provided in series with the low impedance

multiplier input so that only a capacitor from the IAC pin to

GND1 is required to filter noise. The maximum multiplier

output current, which ultimately limits the input line cur-

rent, is set by a resistor from the RSET pin to GND1

according to the formula: IM(MAX) = 3.75V/RSET. Figure 5

shows IM versus IAC for various values of VAOUT. Note that

Figure 5 data was taken with RSET = 15k.

APPLICATIONS INFORMATION

The multiplier output acts as the command signal to the

current loop error amplifier. During steady-state operation

the voltage across RREF = (IM)(RREF) = (IIN)(RS). Based on

this the value for RS is determined by:

(IM(MAX))(RREF)(VIN)(eff)

POUT 2

RS ≤

√

with RSET = 15k, IM(MAX) = 3.75/15k = 250µA. For a 300W

converter with an efficiency (eff) of 0.8 at low line (90VRMS)

and RREF set to 4k, RS should be less than:

(250

µA)(4k)(90VAC)(0.8)

300W 2

= 0.169

Ω

√

A 0.15

Ω resistor will yield a maximum peak input current

of (IM(MAX))(RREF/RS) = (250µA)(4k)/0.15Ω = 6.67A. For

a 100kHz switching frequency with RSET = 15k, CSET = 1.5/

(100kHz)(15k) = 1nF. For added protection the LT1508

provides a second independent current limit comparator.

When the input voltage to the comparator (PKLIM pin) dips

below 0V, GTDR1 pin quickly goes low turning off the PFC

power switch. A resistor divider from VREF to RS (Figure 6)

senses the voltage across the line current sense resistor

(RS) and limits the peak input line current to [(7.5V/R1) +

µA] (R2/RS). The 50µA represents the PKLIM input

current which flows out of the PKLIM pin. With R1 = 10k

and R2 = 1.8k, IIN = 9.6A peak above the 6.67A peak

average plus the input inductor peak ripple current.

Always use RSET to set the primary line current limit. The

PKLIM comparator is only for secondary protection. When

the line current reaches the primary limit, VOUT can no

longer be supported with the given input current and

begins to fall. System stability is maintained by the current

loop which is controlled by the current amplifier. When the

Oscillator Frequency and Maximum

Line Current Setting

The oscillator frequency is set by RSET and CSET. RSET is

the resistor from the RSET pin to GND1 and CSET is the

capacitor from the CSET pin to GND1. RSET should be

determined first. The oscillator frequency, which is equal

to the switching frequency for both the PFC and PWM

section, is determined by:

1.5

(RSET)(CSET)

fOSC =

IAC (µA)

300

150

1508 • F05

250

500

VAOUT = 7V

VAOUT = 6.5V

VAOUT = 6V

VAOUT = 5.5V

VAOUT = 5V

VAOUT = 4.5V

VAOUT = 4V

VAOUT = 3.5V

VAOUT = 3V

VAOUT = 2.5V

Figure 5. Multiplier Current IM vs IAC and VAOUT

–

ILINE

0.15

Ω

IPKLIM

1nF

7.5V

VREF

PKLIM

C1 IS TO REJECT NOISE, CURRENT

LIMIT DELAY IS ABOUT 2

µs

1.8k

10k

–

1508 • F06

LT1508

Figure 6

LT1508CSW Arkusz danych(PDF) 9 Page - Linear Technology

LT1508CSW Arkusz danych(HTML) 9 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Arkusz danych Pobierz

Link URL