Datasheet

MAX8505
Input Capacitor Design
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
RIPPLE
is the input RMS ripple current.
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.
Compensation Design
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.
For customized compensation networks that increase
stability or transient response, the simplified loop gain
can be described by the equation:
where:
gm
ERR
(COMP transconductance) = 100µmho
R
OERR
(output resistance of transconductance
amplifier) = 20MΩ
C
COMP
(compensation capacitor at COMP pin)
R
T
(current-sense transresistance) = 0.086Ω
C
PARA
(parasitic capacitance at COMP pin) = 10pF
R
L
(load resistor)
C
OUT
(output capacitor)
R
ESR
(series resistance of C
OUT
)
s = j2πf
In designing the compensation circuit, select an appro-
priate converter bandwidth (f
C
) to stabilize the system
while maximizing transient response. This bandwidth
should not exceed 1/10 of the switching frequency. Use
100kHz as a reasonable starting point. Calculate
C
COMP
based on this bandwidth using the following
equation:
where R2 and R3 are the feedback resistors.
Calculate C
COMP
to cancel out the pole created by R
L
and C
OUT
using the following equation;
CR
C
R
COMP L
OUT
COMP
=×
R
IRRR fC
Vgm
COMP
OUT T C OUT
OUT ER
=
××+×××
×
()322π
RR
R× 3
A
V
V
gm R
sC R
s
VOL
FB
OUT
ERR OERR
COMP COMP
=× × ×
××+
×
1
(
CCR sCR
R
R
COMP OERR PARA COMP
L
×+××× +
⎛
⎝
⎜
⎞
⎠
⎟
×
11)( )
TT
OUT ESR
OUT L
sC R
sC R
×
××+
××+
⎛
⎝
⎜
⎞
⎠
⎟
1
1
II
VVV
V
RIPPLE LOAD
OUT IN OUT
IN
=×
×−()
2
3A, 1MHz, 1% Accurate, Internal Switch
Step-Down Regulator with Power-OK
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