MAX8505 MAXIM [Maxim Integrated Products], MAX8505 Datasheet - Page 12

• Manufacturer Part Number: MAX8505
• Description: 3A, 1MHz, 1% Accurate, Internal Switch Step-Down Regulator with Power-OK
• Manufacturer: MAXIM [Maxim Integrated Products]

The input capacitor reduces the current peaks drawn
from the input power supply and reduces switching
noise in the IC. The impedance of the input capacitor at
the switching frequency should be less than that of the
input source so high-frequency switching currents do
not pass through the input source but instead are
shunted through the input capacitor. A high source
impedance requires larger input capacitance. The
input capacitor must meet the ripple current require-
ment imposed by the switching currents. The RMS
input ripple current is given by:
where I
Use sufficient input bypass capacitance to ensure that
the absolute maximum voltage rating of the MAX8505 is
not exceeded in any condition. When input supply is
not located close to the MAX8505, a bulk bypass input
capacitor may be needed.
The double pole formed by the inductor and output
capacitor of most voltage-mode controllers introduces
a large phase shift, which requires an elaborate
compensation network to stabilize the control loop.
The MAX8505 controller utilizes a current-mode control
scheme that regulates the output voltage by forcing
the required current through the external inductor,
eliminating the double pole caused by the inductor
and output capacitor, and greatly simplifying the
compensation network. A simple type 1 compensation
with single compensation resistor (R1) and compensa-
tion capacitor (C8) create a stable and high-bandwidth
loop (see the Typical Operating Circuit).
An internal transconductance error amplifier compen-
sates the control loop. Connect a series resistor and
capacitor between COMP (the output of the error amplifi-
er) and GND to form a pole-zero pair. The external
inductor, internal current-sensing circuitry, output capaci-
tor, and external compensation circuit determine the loop
stability. Choose the inductor and output capacitor based
on performance, size, and cost. Additionally, select the
compensation resistor and capacitor to optimize control-
loop stability. The component values shown in the Typical
Operating Circuit yield stable operation over a broad
range of input-to-output voltages.
3A, 1MHz, 1% Accurate, Internal Switch
Step-Down Regulator with Power-OK
12
______________________________________________________________________________________
I
RIPPLE
RIPPLE
is the input RMS ripple current.
=
I
LOAD
×
V
OUT
Input Capacitor Design
Compensation Design
×
(
V
V
IN
IN
2
−
V
OUT
)
For customized compensation networks that increase
stability or transient response, the simplified loop gain
can be described by the equation:
where:
gm
R
amplifier) = 20MΩ
C
R
C
R
C
R
s = j2πf
In designing the compensation circuit, select an appro-
priate converter bandwidth (f
while maximizing transient response. This bandwidth
should not exceed 1/10 of the switching frequency. Use
100kHz as a reasonable starting point. Calculate
C
equation:
where R2 and R3 are the feedback resistors.
Calculate C
and C



A
R
R
OERR
T
L
ESR
COMP
PARA
OUT
COMP
(
VOL
L
s C
T
(current-sense transresistance) = 0.086Ω
(load resistor)
ERR
R
×
×
COMP
(series resistance of C
OUT
(output capacitor)



=
(output resistance of transconductance
(parasitic capacitance at COMP pin) = 10pF
COMP
(COMP transconductance) = 100µmho
s C
(compensation capacitor at COMP pin)
based on this bandwidth using the following
s C
V
×
V
OUT
×
using the following equation;
FB
COMP
=
OUT
I
OUT
×
OUT
×
R
C
s C
gm
COMP
COMP
×
×
to cancel out the pole created by R
×
R
×
ERR
R
R
ESR
COMP
L
T
V
+
×
OUT
+ ×
=
×
+
1
(
1
R
R
) (
R
1
L
×
3



OERR
OUT
×
×
R
+
s C
C
gm
R
R
COMP
×
C
) to stabilize the system
2
COMP
)
OUT
)
ERR
×
×
PARA
2
π
+
×
1
×
R
f
2
×
C
R
×
COMP
C
OUT
+
1
)

 ×

L