
2-116
Figure 5 shows the critical power components of the converter.
To minimize the voltage overshoot the interconnecting wires
indicated by heavy lines should be part of ground or power
plane in a printed circuit board. The components shown in
Figure 6 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 HIP6005B within 3
inches of the MOSFET, Q
1
. The circuit traces for the
MOSFET’s gate and source connections from the HIP6005B
must be sized to handle up to 1A peak current.
Figure 6 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
μ
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.
Feedback Compensation
Figure 7 highlights the voltage-mode control loop for a 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
).
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 frequency 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
V
OSC
.
Modulator Break Frequency Equations
The compensation network consists of the error amplifier
(internal to the HIP6005B) 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 (R
1
, R
2
,
R
3
, C
1
, C
2
, and C
3
) in Figure 8. Use these guidelines for
locating the poles and zeros of the compensation network:
1. Pick Gain (R
2
/R
1
) 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.
4. Place 1
ST
Pole at the ESR Zero.
5. Place 2
ND
Pole at Half the Switching Frequency.
6. Check Gain against Error Amplifier’s Open-Loop Gain.
7. Estimate Phase Margin - Repeat if Necessary.
L
O
C
O
UGATE
PHASE
Q
1
D
2
V
IN
V
OUT
RETURN
HIP6005B
C
IN
L
FIGURE 5. PRINTEDCIRCUITBOARDPOWERANDGROUND
PLANES OR ISLANDS
HIP6005B
SS
GND
V
CC
BOOT
D
1
L
O
C
O
V
OUT
L
Q
1
D
2
PHASE
FIGURE 6. PRINTED CIRCUIT BOARD SMALL SIGNAL
LAYOUT GUIDELINES
+V
IN
C
BOOT
C
VCC
C
SS
+12V
V
OUT
OSC
REFERENCE
L
O
C
O
ESR
V
IN
V
OSC
ERROR
AMP
PWM
DRIVER
(PARASITIC)
FIGURE 7. VOLTAGE-MODE BUCK CONVERTER
COMPENSATION DESIGN
Z
IN
Z
FB
DACOUT
R
1
R
3
R
2
C
3
C
2
C
1
COMP
V
OUT
FB
Z
FB
HIP6005B
Z
IN
COMPARATOR
DETAILED COMPENSATION COMPONENTS
V
E/A
+
-
+
-
+
-
PHASE
F
ESR
O
)
------------------------------------------------
=
F
LC
L
O
x C
O
2
π
x
-----------------------------------------
=
HIP6005B