isl9440b Intersil Corporation, isl9440b Datasheet - Page 21

• Manufacturer Part Number: isl9440b
• Description: Triple Step-down Pwm And Single Linear Controller With Programmable Soft-start
• Manufacturer: Intersil Corporation

Where, R
For this type of modulator, a Type 2 compensation circuit is
usually sufficient.
Figure 41 shows a Type 2 amplifier and its response along
with the responses of the current mode modulator and the
converter. The Type 2 amplifier, in addition to the pole at
origin, has a zero-pole pair that causes a flat gain region at
frequencies in between the zero and the pole.
The zero frequency, the amplifier high-frequency gain, and
the modulator gain are chosen to satisfy most typical
applications. The crossover frequency will appear at the
point where the modulator attenuation equals the amplifier
high frequency gain. The only task that the system designer
has to complete is to specify the output filter capacitors to
position the load main pole somewhere within one decade
lower than the amplifier zero frequency. With this type of
compensation plenty of phase margin is easily achieved due
to zero-pole pair phase ‘boost’.
Conditional stability may occur only when the main load pole
is positioned too much to the left side on the frequency axis
due to excessive output filter capacitance. In this case, the
ESR zero placed within the 1.2kHz to 30kHz range gives
some additional phase ‘boost’. Some phase boost can also
be achieved by connecting capacitor C
upper resistor R
value. Please refer to “Output Capacitor Selection” on
page 23 and “Input Capacitor Selection” on page 24 for
further details.
F
F
Z
P
MODULATOR
G
=
=
M
------------------------------ -
2π R
------------------------------ -
2π R
= 17.5dB
FIGURE 41. FEEDBACK LOOP COMPENSATION
⋅
⋅
O
EA
1
1
F
2
1
is load resistance and C
CONVERTER
PO
⋅
⋅
C
C
1
2
1
=
=
of the divider that sets the output voltage
6kHz
600kHz
F
21
Z
R1
F
O
C
G
is load capacitance.
EA
TYPE 2 EA
Z
in parallel with the
= 18dB
R2
C2
C1
F
ISL9440B, ISL9440C
P
(EQ. 10)
(EQ. 9)
Linear Regulator
The linear regulator controller is a trans-conductance
amplifier with a nominal gain of 2A/V. The N-Channel
MOSFET output buffer can sink a minimum of 50mA.
The reference voltage is 0.8V. With 0V differential at its
input, the controller sinks 21mA of current. For better load
regulation, it is recommended that the resistor from the LDO
input to the base of the PNP (or gate of the PFET) is set so
that the sink current at G4 pin is within 9mA to 31mA over
the entire load and temperature range.
An external PNP transistor or P-Channel MOSFET pass
device can be used. The dominant pole for the loop can be
placed at the base of the PNP (or gate of the PFET), as a
capacitor from emitter-to-base (source to gate of a PFET).
Better load transient response is achieved however, if the
dominant pole is placed at the output with a capacitor to
ground at the output of the regulator.
Under no-load conditions, leakage currents from the pass
transistors supply the output capacitors, even when the
transistor is off. Generally, this is not a problem since the
feedback resistor drains the excess charge. However,
charge may build up on the output capacitor making V
rise above its set point. Care must be taken to insure that the
feedback resistor’s current exceeds the pass transistors
leakage current over the entire temperature range.
The linear regulator output can be supplied by the output of
one of the PWMs. When using a PFET, the output of the
linear regulator will track the PWM supply after the PWM
output rises to a voltage greater than the threshold of the
PFET pass device. The voltage differential between the
PWM and the linear output will be the load current times the
r
Base-Drive Noise Reduction
The high-impedance base driver is susceptible to system
noise, especially when the linear regulator is lightly loaded.
Capacitively coupled switching noise or inductively coupled
EMI onto the base drive causes fluctuations in the base
DS(ON)
.
60
50
40
30
20
10
0
0.79
FIGURE 42. LINEAR CONTROLLER GAIN
0.8
FEEDBACK VOLTAGE (V)
0.81
0.82
0.83
0.84
January 15, 2009
FN6799.0
LDO
0.85