LTM4675 datasheet(13/20 Pages) LINER

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LTC3870-1

38701f

For more information www.linear.com/LTC3870-1

applicaTions inForMaTion

the phase difference. The voltage on the filter network is

adjusted until the phase and frequency of the internal and

external oscillators are identical. At the stable operating

point, the phase detector output is high impedance and

the filter capacitor holds the voltage.

Typically, the external clock (on the SYNC pin) input high

threshold is 2V, while the input low threshold is 0.4V.

Fault Protection and Responses

LTC3887-1 master controllers monitor system voltage,

current, and temperature and provide many protection

features during fault conditions. LTC3870-1 slave con-

trollers do not provide as many fault monitors as master

controllers and have to respond to fault signals from the

master controller. FAULT0 and FAULT1 pins are designed

to share fault signals between masters and slaves. In a

typical parallel application, connect the FAULT pins on

LTC3870-1 to the master GPIO pins of the correspond-

ing paralleled channels and program the master GPIO as

fault sharing, so that the slave controller can respond to

all fault protections from the master. When the FAULT pin

is pulled below 1.4V, the PWM pin in the corresponding

channel is in three-state. When the FAULT pin voltage is

above 2V, the corresponding channel returns to normal

operation. During fault conditions, all internal circuits in

LTC3870-1 are still running so the slave controllers can

immediately go back to normal operation when the FAULT

pin is released.

LTC3870-1 has internal thermal shutdown protection

which forces the PWM pin three-state when the junction

temperature is higher than 160°C. In thermal shutdown,

FAULT0 and FAULT1 pins are also pulled low. There is a

500kΩ pull-down resistor on each FAULT pin which sets

the default voltage on FAULT pins low if FAULT pins are

left floating.

Transient Response and Loop Stability

In a typical parallel operation, LTC3870-1 cooperates with

master controllers to supply more current. To achieve

balanced current sharing between master and slave, it is

recommended that each slave channel copy the design

from the master channel. Select same inductors, same

MOSFET driver, same current sensing circuit and same

output capacitors between the master channel and slave

channels. Control loop and compensation design on the

ITH pin should start with the single phase operation of the

master controller. If the master and slave channels are

exactly the same, then the transient response and loop

stability of the multiphase design is almost the same as

the single phase operation of the master by tying the ITH

pins together between the master and slaves. For example,

designthecompensationforasinglephase1.8V/20Aoutput

using LTC3887-1 with a 0.56µH inductor and 530µF output

capacitors. To extend the output to 1.8V/40A, simply paral-

lel one channel of LTC3870-1 with the same inductor and

output capacitors (total output capacitors are 1060µF) and

tie the ITH pin of LTC3870-1 to the master ITH. The loop

stabilityandtransientresponsesofthetwophaseconverter

areverysimilartothesinglephasedesignwithoutanyextra

compensator on the ITH pin of LTC3870-1 slave controller.

Furthermore, LTpowerCAD is provided on the LTC website

as a free download for transient and stability analysis.

To minimize the high frequency noise on the ITH trace

between master and slave ITH pins, a small filter capacitor

in the range of tens of pF can be placed closely at each ITH

pin of the slave controller. This small capacitor normally

does not significantly affect the closed loop bandwidth but

increases the gain margin at high frequency.

Mode Selection and Pre-Biased Startup

There may be situations that require the power supply to

start up with a pre-bias on the output capacitors. In this

case, it is desirable to start up without discharging the

output capacitors. The LTC3870-1 can be configured to

DCM mode for pre-biased start-up. If a PGOOD signal is

available on the master controller, the PGOOD pin can be

connected to MODE pins of LTC3870-1 to ensure DCM

operation at startup and CCM operation at steady state.

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