
PRODUCT SPECIFICATION
RC5054A
7
P
shown in Figure 3 should be located as close together as
possible. Please note that the capacitors C
IN
and C
O
each
represent numerous physical capacitors. Locate the
RC5054A within 3 inches of the MOSFETs, Q1 and Q2. The
circuit traces for the MOSFETs’ gate and source connections
from the RC5054A must be sized to handle up to 1A peak
current.
Figure 3. Printed Circuit Board
Power and Ground Planes or Islands
Figure 4 shows the circuit traces that require additional
layout consideration. Use single point and ground plane
construction for the circuits shown. Minimize any leakage
current paths on the SS PIN and locate the capacitor, C
SS
close to the SS pin because the internal current source is only
10
m
A. Provide local V
CC
decoupling between V
CC
and
GND pins. Locate the capacitor, C
BOOT
as close as practical
to the BOOT and PHASE pins.
Figure 4. Printed Circuit Board
Small Signal Layout Guidelines
Feedback Compensation
Figure 5 highlights the voltage-mode control loop for a
synchronous-rectified buck converter. The output voltage
(V
OUT
) is regulated to the Reference voltage level. The error
amplifier (Error Amp) output (V
E/A
) is compared with the
oscillator (OSC) triangular wave to provide a pulse-width
modulated (PWM) wave with an amplitude of V
IN
at the
PHASE node. The PWM wave is smoothed by the output
filter (L
O
and C
O
).
Figure 5. Voltage-Mode Buck
Converter Compensation Design
The modulator transfer function is the small-signal transfer
function of V
OUT
/V
E/A
. This function is dominated by a DC
Gain and the output filter (L
O
and C
O
), with a double pole
break freaquency at F
LC
and a zero at F
ESR
. The DC Gain of
the modulator is simply the input voltage (V
IN
) divided by
the peak-to-peak oscillator voltage
D
V
OSC
.
Modulator Break Frequency Equations
The compensation network consists of the error amplifier
(internal to the RC5054A) and the impedance networks Z
IN
and Z
FB
. The goal of the compensation network is to provide
a closed loop transfer function with the highest 0dB crossing
frequency (f
0dB
) and adequate phase margin. Phase margin
is the difference between the closed loop phase at f
0dB
and
180 degrees. The equations below relate the compensation
network’s poles, zeros and gain to the components (R1, R2,
R3, C1, C2, and C3) in Figure 5. Use these guidelines for
locating the poles and zeros of the compensation network:
1.
Pick Gain (R2/R1) for desired converter bandwidth
2.
Place 1
ST
Zero Below Filter’s Double Pole (~75% F
LC
)
3.
Place 2
ND
Zero at Filter’s Double Pole
PGND
L
O
C
O
LGATE
UGATE
PHASE
Q1
Q2
D2
V
IN
V
OUT
RETURN
RC5054A
C
IN
L
+12V
RC5054A
SS
GND
VCC
BOOT
D1
L
O
C
O
V
OUT
Q1
Q2
PHASE
+V
IN
C
BOOT
C
VCC
C
SS
L
V
OUT
OSC
REFERENCE
L
O
C
O
ESR
V
IN
D
V
OSC
ERROR
AMP
PWM
DRIVER
(PARASITIC)
Z
FB
+
-
DACOUT
R1
R3
R2
C3
C2
C1
COMP
V
OUT
FB
Z
FB
RC5054
Z
IN
COMPARATOR
DRIVER
DETAILED COMPENSATION COMPONENTS
PHASE
V
E/A
+
-
+
-
Z
IN
F
LC
L
O
2
p
C
O
·
·
-----------------1
=
F
ESR
p
ESR
C
O
·
·
2
=