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PDF ISL8845A Data sheet ( Hoja de datos )
| Número de pieza | ISL8845A | |
| Descripción | (ISL8840A - ISL8845A) High Performance Industry Standard Single-Ended Current Mode PWM Controller | |
| Fabricantes | Intersil Corporation | |
| Logotipo |  |
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Data Sheet
ISL8840A, ISL8841A, ISL8842A,
ISL8843A, ISL8844A, ISL8845A
April 18, 2007
FN6320.3
High Performance Industry Standard
Single-Ended Current Mode PWM
Controller
The ISL884xA is a high performance drop-in replacement for
the popular 28C4x and 18C4x PWM controllers suitable for a
wide range of power conversion applications including
boost, flyback, and isolated output configurations. Its fast
signal propagation and output switching characteristics
make this an ideal product for existing and new designs.
Features include 30V operation, low operating current, 90μA
start-up current, adjustable operating frequency to 2MHz,
and high peak current drive capability with 20ns rise and fall
times.
PART NUMBER
ISL8840A
ISL8841A
ISL8842A
ISL8843A
ISL8844A
www.DataSheet4U.com
ISL8845A
RISING UVLO
7.0
7.0
14.4V
8.4V
14.4V
8.4V
MAX. DUTY CYCLE
100%
50%
100%
100%
50%
50%
Pinout
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
(8 LD SOIC, MSOP)
TOP VIEW
COMP 1
FB 2
CS 3
RTCT 4
8 VREF
7 VDD
6 OUT
5 GND
Features
• 1A MOSFET gate driver
• 90μA start-up current, 125μA maximum
• 35ns propagation delay current sense to output
• Fast transient response with peak current mode control
• 30V operation
• Adjustable switching frequency to 2MHz
• 20ns rise and fall times with 1nF output load
• Trimmed timing capacitor discharge current for accurate
deadtime/maximum duty cycle control
• 1.5MHz bandwidth error amplifier
• Tight tolerance voltage reference over line, load and
temperature
• ±3% current limit threshold
• Pb-free plus anneal available and ELV, WEEE, RoHS
Compliant
Applications
• Telecom and datacom power
• Wireless base station power
• File server power
• Industrial power systems
• PC power supplies
• Isolated buck and flyback regulators
• Boost regulators
1 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2005, 2006, 2007. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
1 page  
Typical Application - Boost Converter
VIN+
L1
Q1
R8 C10
CR1
R4
C1
VIN-
R1 R2
C4
U1
COMP
FB
CS
VREF
VDD
OUT
RTCT GND
R3
C7
+
C2
+VOUT
C3
RETURN
R5
C9
R7
VIN+
C8
R6
C5
C6
5 Page 
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
Soft-Start Operation
Soft-start must be implemented externally. One method,
illustrated below, clamps the voltage on COMP.
From the small signal current-mode model [1] it can be
shown that the naturally-sampled modulator gain, Fm,
without slope compensation, is in Equation 6.
Fm
=
---------1----------
SnTsw
(EQ. 6)
VREF
D1 R1 COMP
Q1
GND
C1
FIGURE 5. SOFT-START
The COMP pin is clamped to the voltage on capacitor C1
plus a base-emitter junction by transistor Q1. C1 is charged
from VREF through resistor R1 and the base current of Q1.
At power-up C1 is fully discharged, COMP is at ~0.7V, and
the duty cycle is zero. As C1 charges, the voltage on COMP
increases, and the duty cycle increases in proportion to the
voltage on C1. When COMP reaches the steady state
operating point, the control loop takes over and soft start is
complete. C1 continues to charge up to VREF and no longer
affects COMP. During power down, diode D1 quickly
discharges C1 so that the soft start circuit is properly
initialized prior to the next power on sequence.
Gate Drive
The ISL884xA is capable of sourcing and sinking 1A peak
current. To limit the peak current through the IC, an optional
external resistor may be placed between the totem-pole
output of the IC (OUT pin) and the gate of the MOSFET. This
small series resistor also damps any oscillations caused by
the resonant tank of the parasitic inductances in the traces of
the board and the FET’s input capacitance.
Slope Compensation
For applications where the maximum duty cycle is less than
50%, slope compensation may be used to improve noise
immunity, particularly at lighter loads. The amount of slope
compensation required for noise immunity is determined
empirically, but is generally about 10% of the full scale
current feedback signal. For applications where the duty
cycle is greater than 50%, slope compensation is required to
prevent instability.
Slope compensation may be accomplished by summing an
external ramp with the current feedback signal or by
subtracting the external ramp from the voltage feedback
error signal. Adding the external ramp to the current
feedback signal is the more popular method.
where Sn is the slope of the sawtooth signal and Tsw is the
duration of the half-cycle. When an external ramp is added,
the modulator gain becomes:
Fm
=
(---S----n-----+-----S-1----e---)---T----s---w---
=
-------------1--------------
mcSnTsw
(EQ. 7)
where Se is slope of the external ramp and
mc
=
1
+
S-----e--
Sn
(EQ. 8)
The criteria for determining the correct amount of external
ramp can be determined by appropriately setting the
damping factor of the double-pole located at the switching
frequency. The double-pole will be critically damped if the
Q-factor is set to 1, over-damped for Q < 1, and
under-damped for Q > 1. An under-damped condition may
result in current loop instability.
Q
=
------------------------1-------------------------
π(mc(1 – D) – 0.5)
(EQ. 9)
where D is the percent of on time during a switching cycle.
Setting Q = 1 and solving for Se yields
Se
=
Sn
⎛⎛
⎝⎝
1--
π
+
0.5⎠⎞
------1-------
1–D
–
1⎠⎞
(EQ. 10)
Since Sn and Se are the on time slopes of the current ramp
and the external ramp, respectively, they can be multiplied
by tON to obtain the voltage change that occurs during tON.
Ve
=
Vn
⎛⎛
⎝⎝
1-π-
+
0.5⎠⎞
------1-------
1–D
–
1⎠⎞
(EQ. 11)
where Vn is the change in the current feedback signal (ΔI)
during the on time and Ve is the voltage that must be added
by the external ramp.
For a flyback converter, Vn can be solved for in terms of
input voltage, current transducer components, and primary
inductance, yielding
Ve
=
D------⋅---T----S----W-------⋅---V----I--N-----⋅---R-----C----S--
Lp
⎛
⎝
⎛
⎝
1-π-
+
0.5⎠⎞
------1-------
1–D
–
1⎠⎞
V
(EQ. 12)
where RCS is the current sense resistor, fsw is the switching
frequency, Lp is the primary inductance, VIN is the minimum
input voltage, and D is the maximum duty cycle.
11 FN6320.3
April 18, 2007
11 Page | ||
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| PDF Descargar | [ Datasheet ISL8845A.PDF ] | |
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