XRP7740ILB-0X18-F Exar Corporation, XRP7740ILB-0X18-F Datasheet - Page 19

• Manufacturer Part Number: XRP7740ILB-0X18-F
• Description: IC CTRLR PWM/LDO STP-DWN 40TQFN
• Manufacturer: Exar Corporation
• Series: -r

Specifications of XRP7740ILB-0X18-F

• Topology: Step-Down (Buck) Synchronous (4), Linear (LDO) (1)
• Function: Any Function
• Number Of Outputs: 5
• Frequency - Switching: 1.5MHz
• Voltage/current - Output 1: Controller
• Voltage/current - Output 2: Controller
• Voltage/current - Output 3: Controller
• W/led Driver: No
• W/supervisor: No
• W/sequencer: Yes
• Voltage - Supply: 6.5 V ~ 20 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: *
• Package / Case: *
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: 1016-1697

P
The XRP7708 allows the user to set the upper and lower bound for a power good signal per
channel.
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 S
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.
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 S
There are several considerations when choosing the PWM switching frequency.
Minimum on time determines the minimum duty cycle at the specific switching frequency. The
minimum on time for the XRP7708 is 40ns.
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.
© 2011 Exar Corporation
Main Oscillator Frequency
PWM Switching Frequency
Minimum On Time
OWER
SET_SW_FREQUENCY[6:4]
SET_SW_FREQUENCY[2:0]
Main Oscillator Frequency
G
WITCHING
WITCHING
OOD
The
000
001
010
011
100
101
110
111
Ts
F
LAG
SET_PWRG_TARG_MAX_CHx
�������������� �������� ����������% = �������������� ������������ × ������ ������������������ × 100
F
F
REQUENCY
REQUENCY
600KHZ
429KHZ
750KHz
500KHz
375KHz
1.5MHz
1.0MHz
C
48MHz
20.8ns
Q
Q
000
000
ONSIDERATIONS
NA
u
u
a
a
d
d
44.8MHz 41.6MHz 38.4MHz 35.2MHz
467KHZ
400KHZ
C
933KHz
700KHz
560KHz
350KHz
C
1.4MHz
22.3ns
001
001
h
h
NA
a
a
n
n
19/28
n
n
e
e
867KHz
650KHz
520KHz
433KHz
370KHz
325KHz
1.3MHz
24ns
010
010
l
l
register
NA
D
D
SET_SW_FREQUENCY[6:4]
i
i
g
g
i
i
t
t
800KHz
600KHz
480KHz
400KHz
343KHz
300KHz
1.2MHz
a
a
26ns
011
011
NA
l
l
P
P
sets
X
X
W
W
R
R
M
733KHz
550KHz
440KHz
367KHz
314KHz
M
1.1MHz
28.4ns
P
P
100
100
NA
NA
S
S
7
7
the
t
t
7
7
e
e
0
0
p
p
31.25ns
667KHz
500KHz
400KHz
333KHz
1.0MHz
32MHz
8
8
D
D
101
101
upper
NA
NA
NA
o
o
a
a
w
w
n
n
n
d
n
d
28.8Mhz
900KHz
600KHz
450KHz
360KHz
300KHz
34.7ns
C
C
110
110
X
X
NA
NA
NA
bound,
o
o
R
R
n
n
P
P
t
t
Rev. 1.2.0
r
r
7
7
25.6MHz
o
o
800KHz
533KHz
400KHz
320KHz
39ns
7
111
111
7
l
l
NA
NA
NA
NA
l
l
e
e
4
4
the
r
r
0
0
s
s