MP4560 – 2A, 2MHz, 55V STEP-DOWN CONVERTER
The system may have another zero of
importance, if the output capacitor has a large
capacitance and/or a high ESR value. The zero,
due to the ESR and capacitance of the output
capacitor, is located at:
2. Choose the compensation capacitor (C3) to
achieve the desired phase margin. For
applications with typical inductor values, setting
the compensation zero, fZ1, below one forth of the
crossover frequency provides sufficient phase
margin. Determine the C3 value by the following
equation:
1
fESR
=
2π × C2× RESR
4
C3 >
In this case, a third pole set by the compensation
capacitor (C5) and the compensation resistor (R3)
is used to compensate the effect of the ESR zero
on the loop gain. This pole is located at:
2 π×R 3×fC
3. Determine if the second compensation
capacitor (C5) is required. It is required if the
ESR zero of the output capacitor is located at
less than half of the switching frequency, or the
following relationship is valid:
1
fP3
=
2 π×C5×R 3
The goal of compensation design is to shape the
converter transfer function to get a desired loop
gain. The system crossover frequency where the
feedback loop has the unity gain is important.
Lower crossover frequencies result in slower line
and load transient responses, while higher
crossover frequencies could cause system
unstable. A good rule of thumb is to set the
crossover frequency to approximately one-tenth
of the switching frequency.
fS
2
1
<
2π × C2× RESR
If this is the case, then add the second
compensation capacitor (C5) to set the pole fP3 at
the location of the ESR zero. Determine the C5
value by the equation:
C2× RESR
C5 =
R 3
High Frequency Operation
The switching frequency of MP4560 can be
programmed up to 2MHz by an external resistor.
Table 3—Compensation Values for Typical
Output Voltage/Capacitor Combinations
VOUT
(V)
1.8
C2
(µF)
33
R3
(kꢀ)
32.4
C3
(pF)
680
C6
(pF)
None
L (µH)
The minimum on time of MP4560 is about 100ns
(typ). Pulse skipping operation can be seen more
easily at higher switching frequency due to the
minimum on time.
4.7
2.5
4.7 - 6.8
22
26.1
680
None
3.3
5
12
6.8 -10
15 - 22
10
22
33
22
68.1
47.5
16
220
330
470
None
None
2
Since the internal bootstrap circuitry has higher
impedance, which may not be adequate to
charge the bootstrap capacitor during each
(1-D)×Ts charging period, an external bootstrap
charging diode is strongly recommended if the
switching frequency is about 2MHz (see External
To optimize the compensation components for
conditions not listed in Table 3, the following
procedure can be used.
1. Choose the compensation resistor (R3) to set
the desired crossover frequency. Determine the
R3 value by the following equation:
Bootstrap
Diode
section
for
detailed
implementation information).
With higher switching frequencies, the inductive
reactance (XL) of capacitor comes to dominate,
so that the ESL of input/output capacitor
2 π× C 2× fC VOUT
R 3 =
×
GEA × GCS
VFB
Where fC is the desired crossover frequency.
MP4560 Rev. 1.0
11/5/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
13
MPS [ MONOLITHIC POWER SYSTEMS ]
