MP1570 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
2. Choose the compensation capacitor (C3) to
External Bootstrap Diode
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:
An external bootstrap diode may enhance the
efficiency of the regulator, the applicable
conditions of external BST diode are:
z
VOUT=5V or 3.3V; and
VOUT
VIN
z
Duty cycle is high: D=
>65%
In these cases, an external BST diode is
recommended from the output of the voltage
regulator to BST pin, as shown in Fig.2
4
C3 >
2π × R3 × fC
3. Determine if the second compensation
capacitor (C6) is required. It is required if the
ESR zero of the output capacitor is located at
less than half of the 340KHz switching
frequency, or the following relationship is valid:
External BST Diode
IN4148
BST
CBST
MP1570
5V or 3.3V
SW
+
COUT
L
fS
1
2π × C2× RESR
<
2
Figure 2—Add Optional External Bootstrap
Diode to Enhance Efficiency
is the case, then add the second compensation
capacitor (C6) to set the pole fP3 at the location
of the ESR zero. Determine the C6 value by the
equation:
The recommended external BST diode is
IN4148, and the BST cap is 0.1~1µF.
C2 × RESR
C6 =
R3
TYPICAL APPLICATION CIRCUITS
C5
10nF
INPUT
4.75V to 23V
1
2
IN
EN
BS
SW
OUTPUT
2.5V
3A
7
3
MP1570
8
5
SS
FB
GND
COMP
4
6
D1
B130
(optional)
C3
3.3nF
C6
(optional)
MP1570_F03
Figure 3—MP1570 with AVX 47μF, 6.3V Ceramic Output Capacitor
MP1570 Rev. 1.6
10/10/2007
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