ISL88550AIRZ Intersil, ISL88550AIRZ Datasheet - Page 12

• Manufacturer Part Number: ISL88550AIRZ
• Description: IC PWM CONTROLLER 28TQFN
• Manufacturer: Intersil

Specifications of ISL88550AIRZ

• Applications: PWM Controller
• Voltage - Input: 2 ~ 25 V
• Current - Supply: 25µA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TQFN
• Rohs Compliant: YES
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -

Detailed Description
The ISL88550A combines a synchronous buck PWM
controller, an LDO linear regulator, and a 10mA reference
output. The buck controller drives two external N-Channel
MOSFETs to deliver load currents up to 15A and generates
voltages down to 0.7V from a +2V to +25V input. The LDO
Linear Regulator can source up to 2.5A and sink up to -2.0A
continuously. These features make the ISL88550A ideally
suited for DDR memory application.
The ISL88550A buck regulator is equipped with a fixed
switching frequency up to 600kHz constant on-time PWM
architecture. This control scheme handles wide input/output
voltage ratios with ease, and provides 100ns “instant-on”
response to load transients while maintaining high efficiency
with relatively constant switching frequency.
The buck controller (LDO) and buffered reference output are
provided with independent current limits. Lossless fold-back
current limit in the buck regulator is achieved by monitoring
the drain to source voltage drop of the low side FET. The
ILIM input is used to adjust this current limit. Overvoltage
protection is achieved by latching the low side synchronous
FET on and the high side FET off when the output voltage is
over 116% of its set output. It also features an optional
undervoltage protection by latching the MOSFET drivers to
the OFF state during an overcurrent condition when the
output voltage is lower than 70% of the regulated output.
Once the overcurrent condition is removed, the regulator is
allowed to soft-start again. This helps minimize power
dissipation during a short circuit condition.
The current limit in the LDO and buffered reference output is
+3.0A/-2.5A and ±40mA respectively and neither have the
overvoltage or undervoltage protection. When the current
limit in either output is reached, the output no longer
regulates the voltage, but will regulate the current to the
value of the current limit.
+5V Bias Supply (V
The ISL88550A requires an external +5V bias supply in
addition to the input voltage (V
external to the IC improves the efficiency and eliminates the
cost associated with the +5V linear regulator that would
otherwise be needed to supply the PWM circuit and the gate
drivers. V
the input source is a fixed +4.5V to +5.5V supply.
V
drivers, and AV
The current from the AV
supply the current for the IC and the gate drive for the
MOSFET’s. This maximim current can be estimated in
Equation 1:
I
BIAS
DD
=
is the supply input for the Buck regulator’s MOSFET
I
VDD
DD
+
I
AVDD
, AV
DD
+
DD
f
SW
supplies the power for the rest of the IC.
x
and V
(
Q
G
DD
1
DD
+
Q
IN
and AV
G
and V
2
can be connected together if
12
)
IN
). Keeping the bias supply
DD
DD
power supply must
)
(EQ. 1)
ISL88550A
Where I
V
MOSFETs Q
Application Circuit" on page 22, and f
frequency.
Free-Running Constant-ON-Time PWM
The constant ON-time PWM control architecture is a pseudo
fixed frequency, constant on-time, current-mode regulator
with voltage feed forward (Figure 21). This architecture relies
on the output filter capacitor’s ESR to act as a current-sense
resistor, so the output ripple voltage provides the PWM ramp
signal. The control algorithm is simple: the high-side switch
ON-time is determined solely by a one-shot whose pulse
width is inversely proportional to input voltage and directly
proportional to the output voltage. Another one-shot sets a
minimum off-time of 300ns typically. The ON-time one-shot
is triggered if the error comparator is low, the low-side switch
current is below the valley current-limit threshold, and the
minimum off-time one-shot has timed out.
ON-Time One Shot (t
The heart of the PWM core is the one-shot that sets the
high-side switch ON-time. This fast, low-jitter, adjustable
one-shot includes circuitry that varies the ON-time in
response to input and output voltages. The high-side switch
ON-time is inversely proportional to the input voltage (V
and is proportional to the output voltage, as shown in
Equation 2:
where K (the ON-time scale factor) is set by the
connection (Table 1) and r
the synchronous rectifier (Q
Circuit" on page 22. This algorithm results in a nearly
constant switching frequency despite the lack of a fixed
frequency clock generator. The benefits of a constant
switching frequency are two-fold:
The ON-time one-shot has good accuracy at the operating
points specified in the “Electrical Specifications” table
(approximately ±12.5% at 600kHz and 450kHz and ±10% at
200kHz and 300kHz) on page 3. ON-times at operating
points far removed from the conditions specified in the
“Electrical Specifications” table on page 3 can vary over a
wider range. For example, the 600kHz setting typically runs
approximately 10% slower with inputs much greater than 5V
due to the very short ON-times required.
The constant ON-time translates only roughly to a constant
switching frequency. The ON-times guaranteed in the
t
on
1. The frequency can be selected to avoid noise-sensitive
2. The inductor ripple-current operating point remains
DD;
=
regions such as the 455kHz IF band.
relatively constant, resulting in an easy design
methodology and predictable output voltage ripple.
K
AV
×
VDD
(
DD,
V
OUT
Q
1
+ I
+
G1
and Q
I
LOAD
AVDD
V
and Q
IN
×
r
2
DS
are the quiescent supply currents into
(at V
(
ON
G2
ON
)
Q
2
DS(ON)Q2
are the total gate charges of
GS
)
2
)
) in the "Typical Application
= 5V) in the "Typical
is the ON-resistance of
SW
is the switching
t
ON
April 23, 2008
input
FN6168.3
(EQ. 2)
IN
)