XRP7714_1009 Datasheet, PDF (19/29 Pages) Exar Corporation – Quad Channel Digital PWM Step Down Controller

XRP7714
Quad Channel Digital PWM Step Down Controller
SOFT-STOP
The SET_PD_FALL_CHx register is a 16 bit register. This register specifies the soft-stop delay and
ramp (fall-time) characteristics for when the chip receives a channel disable indication from the
Host to shutdown the channel.
Bits [15:10] specify the delay after disabling a channel but before starting the shutdown of the
channel; where each bit represents 250µs steps. Bits [9:0] specify the fall time of the channel;
these 10 bits define the number of microseconds for each 50mV increment to reach the discharge
threshold.
Enable
Signal
Vout
DELAY
Fall Time
Bit [10:15]
PD_DELAY_CHx
REGISTER
Bit [0:9]
Fig. 25: Channel Soft-Stop Sequence
POWER GOOD FLAG
The XRP7714 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 XRP7714.
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
© 2010 Exar Corporation
19/29
Rev. 1.1.4