LT7170 datasheet(23/36 Pages) AD

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Data Sheet

LT7170/LT7170-1

APPLICATIONS INFORMATION

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Rev. 0 | 23 of 36

The LT7170/LT7170-1 automatically synchronize PWM switching

to an external clock input on the SYNC/PWM_CFG pin unless

the LT7170/LT7170-1 are configured as an output driver or are

programmed to ignore the input clock. When an external synchro-

nization clock is used on the SYNC/PWM_CFG pin, the LT7170/

LT7170-1 automatically use forced continuous mode. The LT7170/

LT7170-1 continue PWM operation using their own internal oscilla-

tor if the external clock signal is lost. If an external synchroniza-

tion clock is used, it is recommended to program the FREQUEN-

CY_SWITCH command or to use RPWM_CFG to set the internal

oscillator frequency to a value close to the external clock frequency

to ensure that the PWM fSW remains consistent if the external clock

is lost. The LT7170/LT7170-1 can be programmed to ignore an

external clock by writing a 1 to MFR_SYNC_CONFIG_ LT7170, Bit

For an input clock frequency of 400 kHz to 625 kHz, it is recom-

mended to use a RPWM_CFG that selects a PWM frequency of 500

kHz. For an input clock frequency from 625 kHz to 1.25 MHz, it is

recommended to use a RPWM_CFG that selects a PWM frequency of

1 MHz. For an input clock frequency from 1.25 MHz to 2.5 MHz, it

is recommended to use a RPWM_CFG that selects a PWM frequency

of 2 MHz. For an input clock frequency from 2.5 MHz to 4 MHz, it is

recommended to use a RPWM_CFG that selects a PWM frequency of

4 MHz.

Figure 34. PWM Resistor Configuration Without an External Clock

The LT7170/LT7170-1 can be configured to provide a synchronizing

clock output on the SYNC/PWM_CFG pin to other devices by

setting Bit 0 of MFR_SYNC_CONFIG_LT7170 to 1.

If the SYNC/PWM_CFG output clock is enabled, the LT7170/

LT7170-1 drive the SYNC/PWM_CFG pin as a square wave

from 0 V to 1.88 V (typical) at the frequency programmed in

the FREQUENCY_SWITCH command. The phase of SYNC leads

the phase of the Phase 0 PWM output by the value set in the

MFR_PWM_PHASE_ LT7170 command. Only one device connect-

ed to the SYNC/PWM_CFG pin can be configured as an output.

When a clock is active on the SYNC/PWM_CFG pin, the

MFR_PWM_PHASE_LT7170 command specifies the phase rela-

tionship between the rising edge of the SYNC/PWM_CFG pin and

the rising edge of the LT7170/LT7170-1 SW0 pin.

Figure 35. PWM Resistor Configuration with an External Clock

If both the SYNC/PWM_CFG RPWM_CFG and synchronization clock

input functions are used, the clock signal must be AC-coupled

using a 1.5 nF capacitor between the clock source and the LT7170/

LT7170-1, as shown in Figure 35. In this case, the clock signal

must be inactive during LT7170/LT7170-1 initialization to ensure

it does not interfere with the resistor configuration function. If the

AC-coupled clock source output impedance is less than 50 Ω, a 50

Ω resistor must be added in-series with the clock source. See Table

8 for PWM_CFG resistor selections.

If the SYNC/PWM_CFG pin is used only as a clock input or output,

and MFR_CONFIG_ALL_LT7170, Bit 6, is written to 1 in the NVM

to disable the resistor configuration pins, no configuration resistor or

AC-coupling capacitor is required.

If an external clock is applied to the SYNC/PWM_CFG pin through-

out initialization, and the function of the resistor configuration pins

is not disabled, the LT7170/LT7170-1 measure the clock frequency

and initialize the FREQUENCY_SWITCH command to the meas-

ured frequency rounded to the nearest 100 kHz. In this case,

MFR_PWM_PHASE is set to 0° and forced continuous mode is

selected. Note that, unless the functionality of the resistor config-

uration pins is disabled, an external clock applied to the SYNC/

PWM_CFG pin must be either active or inactive throughout the

entire LT7170/LT7170-1 initialization process. If the clock activity

changes during initialization, for example, if the clock starts after

initialization begins but before it completes, the frequency measure-

ment may be inaccurate, which can lead to the LT7170/LT7170-1

incorrectly initializing the FREQUENCY_SWITCH command or de-

claring a pin configuration fault. See the MFR_PIN_CONFIG_STA-

TUS command in the LT7170/LT7170-1 PMBus/I2C Reference Man-

ual for more information regarding pin configuration faults.

SINGLE PHASE FOR THE LT7170

The LT7170 is a single-phase, monolithic, DC/DC synchronous,

step-down regulator that delivers up to 20 A of continuous output

current. A single phase drives the SW0 and SW1 pins tied to one

inductor to drive a single-regulated output supply.

For single-phase operation, connect the LT7170 SW0 and SW1

pins together and to a single inductor.

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