LT3592IMSE datasheet(18/24 Pages) LINEAR

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LT3592

3592fc

APPLICATIONS INFORMATION

ground the SHDN pin, the SW pin current will drop to es-

sentially zero. However, if the VIN pin is grounded while

the output is held high, then parasitic diodes inside the

LT3592 can pull large currents from the output through

the SW pin and the VIN pin. Figure 8 shows a circuit that

will run only when the input voltage is present and that

protects against a shorted or reversed input.

Hot Plugging Safely

The small size, robustness, and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT3592 circuits. However, these capaci-

tors can cause problems if the LT3592 is plugged into a

live supply (see Linear Technology Application Note 88 for

a complete discussion). The low loss ceramic capacitor

combined with stray inductance in series with the power

source forms an underdamped tank circuit, and the volt-

age at the VIN pin of the LT35392 can ring to twice the

nominal input voltage, possibly exceeding the LT3592’s

rating and damaging the part. If the input supply is poorly

controlled or the user will be plugging the LT3592 into an

energized supply, the input network should be designed

to prevent this overshoot.

Figure 9 shows the waveforms that result when an LT3592

circuit is connected to a 32V supply through six feet of 24

gauge twisted pair. The ﬁrst plot is the response with a 1μF

ceramic capacitor at the input. The input voltage rings as

high as 56V and the input current peaks at 16A.

One method of damping the tank circuit is to add another

capacitor with a series resistor to the circuit. In Figure 9b,

a tantalum chip capacitor has been added. This capacitor’s

high equivalent series resistance (ESR) damps the circuit

and eliminates the voltage overshoot. The extra capacitor

improves low frequency ripple ﬁltering and can slightly

improve the efﬁciency of the circuit, thought it is likely

to be the largest component in the circuit. An alternate

solution is shown in Figure 9c. A 1Ω resistor is added in

series with the input to eliminate the voltage overshoot

(it also reduces the peak input current). A 0.1μF capacitor

improves high frequency ﬁltering. This solution is smaller

and less expensive than the tantalum capacitor. For high

input voltages, the impact of the 1Ω resistor on efﬁciency

is minor, reducing it by less than one half percent for a two

red series LED load in BRIGHT mode operating from 32V.

Frequency Compensation

The LT3592 uses current mode control to regulate the

loop, whether the current control or voltage control loop

is active. This simpliﬁes loop compensation. In particular,

the LT3592 does not require the ESR of the output capaci-

tor for stability, allowing the use of ceramic capacitors

to achieve low output ripple and small circuit size. A low

ESR output capacitor will typically provide for a greater

margin of circuit stability than an otherwise equivalent

capacitor with higher ESR, although the higher ESR will

tend to provide a faster loop response. Figure 10 shows

an equivalent circuit for the LT3592 control loops, both for

current and voltage mode. Both use the same error ampliﬁer

and power section, but an additional voltage gain amp is

used in conjuction with the external current sense resistor

to implement output current control. The error ampliﬁer is

a transconductance type with ﬁnite output impedance. The

power section, consisting of the modulator, power switch,

and inductor, is modeled as a transconductance ampliﬁer

generating an output current proportional to the voltage

at the VC node. Note that the output capacitor integrates

this current, and that the capacitor on the VC node (CC)

integrates the error ampliﬁer output current, resulting in

Figure 10. Model for Loop Response

3592 F10

ESR

0.7V

GND

gm = 0.7A/V

300k

gm = 300μA/V

gm = 1/5k

BRIGHT

CAP

OUT

VFB

1.2V

RSENSE

30k

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