LT8331 datasheet(18/24 Pages) AD

18 / 24 page

LT8337/LT8337-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

bulk capacitance may be necessary. This can be provided

with an electrolytic capacitor. When choosing a capacitor,

special attention should be given to capacitor's data sheet

to calculate the effective capacitance under the relevant

operating conditions of voltage bias and temperature. A

physically larger capacitor, or one with a higher voltage

rating, may be required. For good starting values, refer

to the Typical Applications section.

Output Power Good (LT8337 Only)

When the LT8337’s FB voltage is within the ±8% window

of the regulation point, the output voltage is considered

good and the open-drain PG pin goes high impedance

and is typically pulled high with an external resistor.

Otherwise, the internal pull-down device will pull the PG

pin low. To prevent glitching both the upper and lower

thresholds include 1% of hysteresis. The PG pin is also

actively pulled low during several fault conditions: corre-

sponding EN/UV pin below 1V, INTVCC voltage falling too

low, VIN under voltage, or thermal shutdown.

Frequency Compensation (LT8337-1 Only)

The LT8337-1 has a VC pin which can be used to opti-

mize the loop compensation. Designing the compensation

network is a bit complicated and the best values depend

on the application and in particular the type of output

capacitor. A practical approach is to start with one of the

circuits in the data sheet that is similar to your appli-

cation and tune the compensation network to optimize

the performance. LTspice® simulations can help in this

process. Stability should then be checked across all oper-

ating conditions, including load current, input voltage,

and temperature.

Figure 4 shows an equivalent circuit for the LT8337-1

control loop. The error amplifier is a transconductance

amplifier with finite output impedance. The power section,

consisting of the modulator, power switches, and inductor,

is modeled as a transconductance amplifier generating an

output current proportional to the voltage at the VC pin.

Note that the output capacitor integrates this current and

that the capacitor on the VC pin (CC) integrates the error

amplifier output current, resulting in two poles in the loop.

A zero is required and comes from a resistor RC in series

with CC. This simple model works well as long as the value

of the inductor is not too high and the loop crossover

frequency is much lower than the switching frequency.

A small capacitor CF can be added to filter the switch-

ing noise that is coupled on the VC pin. A phase lead

capacitor (CPL) across the feedback divider can be used

to improve the transient response and is required to

cancel the parasitic pole caused by the feedback node to

ground capacitance.

Figure 5a shows the transient response for the front page

application with LT8337 which uses internal compensation.

Figure 5b shows a faster transient response of the same

application with LT8337-1 when a 100k RC and 220pF

CC compensation network is used on its VC pin. The

LT8337-1 VIN pin draws 20μA more quiescent current

compared to LT8337.

Figure 4. Mode for Loop Response

LT8337-1

83371 F04

INPUT

OUTPUT

CIN

CURRENT

MODE POWER

STAGE

gm = 6S • (1–D)

1MΩ

gm = 0.4mS

CPL

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