XRP6124 Datasheet, PDF (7/12 Pages) Exar Corporation – Non-Synchronous PFET Step-Down Controller

XRP6124
Non-Synchronous PFET Step-Down Controller
THEORY OF OPERATION
THEORY OF OPERATION
The XRP6124 utilizes a proprietary Constant
On-Time (COT) control with emulated ESR.
The on-time is internally set and automatically
adjusts during operation, inversely with the
voltage V_IN, in order to maintain a constant
frequency. Therefore the switching frequency
is independent of the inductor and capacitor
size, unlike hysteretic controllers. The
emulated ESR ramp allows the use of ceramic
capacitors for output filtering.
At the beginning of each cycle, the XRP6124
turns on the P-Channel FET for a fixed
duration. The on-time is internally set and
adjusted by V_IN. At the end of the on-time the
FET is turned off, for a predetermined
minimum off time T_OFF-MIN (nominally 250ns).
After the T_OFF-MIN has expired the voltage at
feedback pin FB is compared to a voltage
ramp at the feedback comparators positive
input. Once V_FB drops below the ramp voltage,
the FET is turned on and a new cycle starts.
This voltage ramp constitutes an emulated
ESR and makes possible the use of ceramic
capacitors, in addition to other capacitors, as
output filter for the buck converter.
VOLTAGE OPTIONS
The XRP6124 is available in two voltage
options as shown in table 1. The low-voltage
and high-voltage options have T_ON of 0.5µs at
12V_IN and 24V_IN respectively. Note that T_ON is
inversely proportional to V_IN. The constant of
proportionality K, for each voltage option is
shown in table 1. Variation of T_ON versus V_IN is
shown graphically in figures 6 and 7.
Voltage
rating (V)
Part Number
T_ON (µs)
K=T_ONxV_IN
(μs.V)
3-18
XRP6124ES0.5-F 0.5 @ 12V_IN
6
4.5-30 XRP6124HVES0.5-F 0.5 @ 24V_IN
12
Table 1 : XRP6124 voltage options
For a buck converter the switching frequency
fs can be expressed in terms of V_IN, V_OUT and
T_ON as follows:
fs = ^VOUT
V_IN ×T_ON
Since for each voltage option, the product of
V_IN and T_ON is the constant K shown in table 1,
then switching frequency is determined by
V_OUT as shown in table 2.
V_OUT
1.2
1.5
1.8
2.5
3.3
5.0
12
Switching frequency fs(kHz)
XRP6124ES0.5-F
200
250
300
417
550
833
---
XRP6124HVES0.5-F
100
125
150
208
275
417
1000
Table 2: Switching frequency fs
for the XRP6124 voltage options
Where it is advantageous, the high-voltage
option may be used for low-voltage
applications. For example a 12V_IN to 5V_OUT
conversion using a low-voltage option will
result in switching frequency of 833kHz as
shown in table 2. If it is desired to increase
the converter efficiency, then switching losses
can be reduced in half by using a high-voltage
option operating at a switching frequency of
417kHz.
V_OUT
1.2
1.5
1.8
2.5
3.3
5.0
12
Maximum Output Current I_OUT(A)
XRP6124ES0.5-F
XRP6124HVES0.5-F
3.3V_IN
5
5
5
4
---
---
---
5.0V_IN
5
5
5
4
4
---
---
12V_IN
4
4
4
4
3
3
---
18V_IN
---
4
4
4
4
3
2
24V_IN
---
---
4
4
4
3
2
Table 3: Maximum recommended I_OUT
SHORT-CIRCUIT PROTECTION
The purpose of this feature is to prevent an
accidental short-circuit at the output from
damaging the converter. The XRP6124 has a
short-circuit comparator that constantly
monitors the feedback node, after soft-start is
© 2011 Exar Corporation
7/12
Rev. 1.1.0