ISL1902 datasheet(21/25 Pages) RENESAS

21 / 25 page

ISL1902

FN7981 Rev 2.00

Page 21 of 25

March 20, 2013

The linear amplifier may also be used to measure and scale the

LED current directly rather than using the differential current

sensing inputs, CS+ and CS-, that measure the switching current.

Amplifying the signal allows a smaller sensing resistor value for

improved efficiency. As shown in Figure 29, the voltage across

R4 is scaled by the linear amplifier with a gain of 1 + R2/R1.

Control Loop

The control loop configuration is user adjustable with selection of

the external compensation components. For applications

requiring power factor correction (PFC), a very low bandwidth

integrator is used, typically 20Hz or less. In other applications,

the control loop bandwidth can be increased as required, like any

other externally compensated voltage mode PWM controller.

The ISL1902 has two error amplifiers that share a common

non-inverting input and a common output. Each EA can sink

current, but has negligible sourcing capability. An external pull-up

resistor to VREF is required. This configuration causes the EA with

lowest output to be dominant. EA1 is the principal error amplifier

and is compensated externally for low bandwidth for PFC

applications. The downside to a low bandwidth amplifier is that it

cannot respond to input transients quickly. This is where the

second EA comes in. It can be configured for a much higher

bandwidth so that transient response is greatly improved. Under

normal operating conditions EA2 is not active. Its feedback

network is set for a higher output than EA1. When an input surge

occurs, EA1 cannot respond rapidly and the surge propagates to

the output. EA2 becomes active when its feedback voltage

exceeds the reference setpoint and acts to reduce the output

transient. The difference in setpoint is accomplished by

weighting the feedback networks to the EAs appropriately.

The voltage on IOUT is a scaled version of the CS+/CS-

differential signal, having been amplified by 4x. When averaged,

it is a scaled representation of the converter output current, IO.

By measuring IOUT in this manner, both average and

instantaneous inductor currents are known. The instantaneous

inductor current information informs the critical conduction

mode (CrCM) oscillator when the switching current has decayed

to zero.

Figure 30 shows a typical configuration for the control loop. The

sensing resistor RS determines the amplitude of the CS+ signal.

At maximum load this signal must be scaled to match the 0.5V

maximum reference. Since IOUT is 4x the amplitude of the CS+

signal, a simple resistor divider with filtering is required to scale

IOUT prior to connecting to the FB input.

where AIOUT is the IOUT buffer gain (nominally 4x), ADIVIDER is

the gain of the external resistor divider on IOUT (R2/(R1 + R2)),

VREF is the maximum reference level (0.5V), and IO is the

maximum output current. In most applications, RS will be sized

to minimize power dissipation while providing adequate signal

level. The minimum value of the IORS product is 125mV, required

to achieve 0.5V on IOUT.

FIGURE 28. SECOND LED STRING CONTROL

LO UT

LREF

LFB

LPO U T

REFIN

ISL1902

FIGURE 29. DIRECT LED CURRENT SENSING

LOUT

LREF

LFB

FB1

ISL1902

R3 = R1||R2

VREF

AIOUT ADIVIDER IO

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(EQ. 29)

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