XRP7708_12 Datasheet, PDF (19/28 Pages) Exar Corporation – Quad Channel Digital PWM Step Down Controllers

XRP7708 and XRP7740
Quad Channel Digital PWM Step Down Controllers
POWER GOOD FLAG
The XRP7708 allows the user to set the upper and lower bound for a power good signal per
channel. The SET_PWRG_TARG_MAX_CHx register sets the upper bound, the
SET_PWRG_TARG_MIN_CHx register sets the lower bound. Each register has a 20mV LSB
resolution. When the output voltage is within bounds the power good signal is asserted high.
Typically the upper bound should be lower than the over-voltage threshold. In addition, the power
good signal can be delayed by a programmable amount set in the SET_PWRGD_DLY_CHx register.
The power good delay is only set after the soft-start period is finished. If the channel has a pre-
charged condition that falls into the power good region, a power good flag is not set until the soft-
start is finished.
PWM SWITCHING FREQUENCY
The PWM switching frequency is set by choosing the corresponding oscillator frequency and clock
divider ratio in the SET_SW_FREQUENCY register. Bits [6:4] set the oscillator frequency and bits
[2:0] set the clock divider. The tables below summarize the available Main Oscillator and PWM
switching frequency settings in the XRP7708.
Main Oscillator Frequency
SET_SW_FREQUENCY[6:4]
Main Oscillator Frequency
Ts
000
48MHz
20.8ns
001
010
011
100
44.8MHz 41.6MHz 38.4MHz 35.2MHz
22.3ns 24ns
26ns 28.4ns
101
32MHz
31.25ns
110
28.8Mhz
34.7ns
111
25.6MHz
39ns
PWM Switching Frequency
SET_SW_FREQUENCY[6:4]
SET_SW_FREQUENCY[2:0]
000
001
010
011
100
101
110
111
000
NA
1.5MHz
1.0MHz
750KHz
600KHZ
500KHz
429KHZ
375KHz
001
NA
1.4MHz
933KHz
700KHz
560KHz
467KHZ
400KHZ
350KHz
010
NA
1.3MHz
867KHz
650KHz
520KHz
433KHz
370KHz
325KHz
011
NA
1.2MHz
800KHz
600KHz
480KHz
400KHz
343KHz
300KHz
100
NA
1.1MHz
733KHz
550KHz
440KHz
367KHz
314KHz
NA
101
NA
1.0MHz
667KHz
500KHz
400KHz
333KHz
NA
NA
110
NA
900KHz
600KHz
450KHz
360KHz
300KHz
NA
NA
111
NA
800KHz
533KHz
400KHz
320KHz
NA
NA
NA
There are a number of options that could result in similar PWM switching frequency as shown
above. In general, the chip consumes less power at lower oscillator frequency. When
synchronization to external clock is needed, the user can choose the oscillator frequency to be
within +/- 5% of the external clock frequency. A higher Main Oscillator frequency will not improve
accuracy or any performance efficiency.
PWM SWITCHING FREQUENCY CONSIDERATIONS
There are several considerations when choosing the PWM switching frequency.
Minimum On Time
Minimum on time determines the minimum duty cycle at the specific switching frequency. The
minimum on time for the XRP7708 is 40ns.
𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝐷𝑢𝑡𝑦 𝐶𝑦𝑐𝑙𝑒% = 𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝑜𝑛𝑡𝑖𝑚𝑒 × 𝑃𝑊𝑀 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 × 100
As an example the minimum duty cycle is 4% for 1MHz PWM frequency. This is important since the
minimum on time dictates the maximum conversion ratio that the PWM controller can achieve.
© 2012 Exar Corporation
19/28
Rev. 1.2.2