LT7170 datasheet(23/36 Pages) AD

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Data Sheet

LT7170/LT7170-1

APPLICATIONS INFORMATION

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Rev. A | 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 PWM configuration and synchronization clock input func-

tions 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 measured frequency rounded to the nearest 100 kHz. In

this case, MFR_PWM_PHASE is set to 0°. Note that, unless

the functionality of the resistor configuration 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 initializa-

tion process. If the clock activity changes during initialization, for

example, if the clock starts after initialization begins but before it

completes, the frequency measurement may be inaccurate, which

can lead to the LT7170/LT7170-1 incorrectly initializing the FRE-

QUENCY_SWITCH command or declaring a pin configuration fault.

See the MFR_PIN_CONFIG_STATUS command in the LT7170/

LT7170-1 PMBus/I2C Reference Manual for more information re-

garding 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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