XRP7740ILB-0X18-F Exar Corporation, XRP7740ILB-0X18-F Datasheet - Page 20

• 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

The maximum duty cycle is dictated by the minimum required time to sample the current when the
low side MOSFET is on. For the XRP7708, the minimum required sampling time is about 16 clock
cycles at the main oscillator frequency. When calculating maximum duty cycle, the sampling time
needs to be subtracted using the below equation. For example, if operating at 1MHz using the
32MHz main oscillator frequency, the maximum duty cycle would be:
On the other hand, if the 48MHz main oscillator frequency was chosen for the 1MHz PWM
frequency, the maximum duty cycle would be:
Therefore, it is best to choose the highest main oscillator frequency for a particular PWM frequency
if duty cycle limit might be encountered. The maximum duty cycle for any PWM frequency can
easily be determined using the following table:
It is highly recommended that the maximum duty cycle obtained from the table above be
programmed into each of the channels using the SET_DUTY_LIMITER_CHx register. This ensures
that under all conditions (including faults), there will always be sufficient sampling time to measure
the output current. When the duty cycle limit is reached, the output voltage will no longer regulate
and will be clamped based on the maximum duty cycle limit setting.
The PWM Switching frequency plays an important role on overall power conversion efficiency. As
the switching frequency increase, the switching losses also increase. Please see the APPLICATION
INFORMATION, Typical Performance Data for further examples.
Typically the components become smaller as the frequency increases, as long as the ripple
requirements remain constant. At higher frequency the inductor can be smaller in value and have a
smaller footprint while still maintaining the same current rating.
F
The user of the XRP7708 can choose to use an external source as the primary clock for the
XRP7708. This function can be configured using the SET_SYNC_MODE_CONFIG register. This
register sets the operation of the XRP7708 when an external clock is required. By selecting the
© 2011 Exar Corporation
Maximum Duty Cycle
Efficiency
Component Selection and Frequency
REQUENCY
Maximum Duty Cycle
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400KHZ
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933KHz
700KHz
560KHz
350KHz
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1.4MHz
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16
16
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20/28
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370KHz
325KHz
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Main Oscillator Frequency
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800KHz
600KHz
480KHz
400KHz
343KHz
300KHz
1.2MHz
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W
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550KHz
440KHz
367KHz
314KHz
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1.1MHz
P
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NA
S
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7
7
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t
7
7
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0
0
p
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667KHz
500KHz
400KHz
333KHz
1.0MHz
32MHz
8
8
D
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NA
NA
o
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a
a
w
w
n
n
n
d
n
d
28.8Mhz
900KHz
600KHz
450KHz
360KHz
300KHz
C
C
X
X
NA
NA
o
o
R
R
n
n
P
P
t
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Rev. 1.2.0
r
r
7
7
25.6MHz
o
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800KHz
533KHz
400KHz
320KHz
7
7
l
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NA
NA
NA
l
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e
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4
4
r
r
0
0
s
s