LT8650SPJVPBF datasheet(21/34 Pages) AD

21 / 34 page

LT8650SP

Rev. B

For more information www.analog.com

8650sp F03

OUTPUT

LT8650SP

CURRENT MODE

POWER STAGE

1.25M

gm = 0.9mS

gm = 12S

CPL

0.8V

APPLICATIONS INFORMATION

Frequency Compensation

The LT8650SP has VC pins which can be used to optimize

the loop compensation of each channel. If the VC pins are

shorted to VCC, then internal compensation is used. This

simplifies the circuit design and minimizes the quiescent

current, but since the internal compensation has to be

stable across the 300kHz to 3MHz range of switching

frequencies, the internal compensation will not be opti-

mal, especially at high switching frequencies. If the best

transient response is desired, an external compensation

network can be connected to the VC pin, which usually

consists of a series resistor and capacitor (see RC and CC

in the Block Diagram).

Designing the compensation network is a bit complicated

and the best values depend on the application and in par-

ticular 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 application and tune the compensation

networktooptimizetheperformance.LTspice®simulations

can help in this process. Stability should then be checked

across all operating conditions, including load current,

input voltage, and temperature.

Figure 3 shows an equivalent circuit for the LT8650SP

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

phase lead capacitor (CPL)acrossthefeedbackdividercan

be used to improve the transient response and is required

to cancel the parasitic pole caused by the feedback node

to ground capacitance.

Figure 4a shows the transient response for the front page

application which uses internal compensation. Figure 4b

shows the improved transient response of the same ap-

plication when a 14kΩ RC and 220pF CC compensation

network is used. Use of an external compensation network

increasesthequiescentcurrentbyabout50µAperchannel.

Figure 3. Model for Loop Response

Figure 4. Transient Response

(a)

(b)

20µs/DIV

VOUT

100mV/DIV

ILOAD

2A/DIV

2A TO 4A TRANSIENT

3.3VOUT

COUT = 47µF ×2

FCM, fSW = 2MHz

LT8650sp F04a

20µs/DIV

VOUT

100mV/DIV

ILOAD

2A/DIV

2A TO 4A TRANSIENT

3.3VOUT

COUT = 47µF ×2

FCM, fSW = 2MHz

CC = 220pF, RC = 14k

LT8650sp F04b

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