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ISL6559CR-T Datasheet(PDF) 11 Page - Intersil Corporation

Part # ISL6559CR-T
Description  Multi-Phase PWM Controller
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Manufacturer  INTERSIL [Intersil Corporation]
Direct Link  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

ISL6559CR-T Datasheet(HTML) 11 Page - Intersil Corporation

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11
FN9084.8
December 29, 2004
Operation Initialization
Before converter operation is initialized, proper conditions
must exist on the enable and disable inputs. Once these
conditions are met, the controller begins a soft-start interval.
Once the output voltage is within the proper window of
operation, the PGOOD output changes state to update an
external system monitor.
Enable and Disable
The PWM outputs are held in a high-impedance state to
assure the drivers remain off while in shutdown mode. Four
separate input conditions must be met before the ISL6559 is
released from shutdown mode.
First, the bias voltage applied at VCC must reach the internal
power-on reset (POR) circuit rising threshold. Once this
threshold is met, the EN input signal becomes the gate for
soft-start initialization. Hysteresis between the rising and
falling thresholds insures that once enabled, the ISL6559 will
not inadvertently turn off unless the bias voltage drops
substantially. See Electrical Specifications for specifics on
POR rising and falling thresholds.
Second, the ISL6559 features an enable input (EN) for
power sequencing between the controller bias voltage and
another voltage rail. The enable comparator holds the
ISL6559 in shutdown until the voltage at EN rises above
1.23V. The enable comparator has about 90mV of hysteresis
to prevent bounce. It is important that the driver ICs reach
their POR level before the ISL6559 becomes enabled. The
schematic in Figure 7 demonstrates sequencing the ISL6559
with the HIP660X family of Intersil MOSFET drivers which
require 12V bias.
Third, the frequency select\disable input (FS/DIS) will
shutdown the converter when pulled to ground. Under this
condition, the internal oscillator is disabled. The oscillator
resumes operation upon release of FS/DIS and a soft-start
sequence is initiated.
The 11111 VID code is reserved as a signal to the controller
that no load is present. The controller will enter shutdown
mode after receiving this code and will start up upon
receiving any other code. This code is not intended as a
means of enabling the controller when a load is present.
To enable the controller, VCC must be greater than the POR
threshold; the voltage on EN must be greater than 1.23V;
FS/DIS must not be grounded; and VID cannot be equal to
11111. Once these conditions are true, the controller
immediately initiates a soft-start sequence.
Soft-Start
The soft-start time, tSS, is determined by an 11-bit counter
that increments with every pulse of the phase clock. For
example, a converter switching at 250kHz per phase has a
soft-start time of
During the soft-start interval, the soft-start voltage, VRAMP,
increases linearly from zero to 140% of the programmed
DAC voltage. At the same time a current source, IRAMP, is
decreasing from 160
µA down to zero. These signals are
connected as shown in Figure 8 (IOUT may or may not be
connected to FB depending on the particular application).
The ideal diodes in Figure 8 assure that the controller tries to
regulate its output to the lower of either the reference voltage
or VRAMP. Since IRAMP creates an initial offset across RFB of
(RFB x 160µA), the first PWM pulse will not be seen until
VRAMP is greater than the RFB IRAMP offset. This produces a
delay after the ISL6559 enables before the output voltage
starts moving. For example, if VID = 1.5V, RFB = 1kΩ and TSS
= 8.3ms, the delay time can be expressed using Equation 10.
FIGURE 7. POWER SEQUENCING USING THRESHOLD-
SENSITIVE ENABLE (EN) FUNCTION
-
+
1.23V (± 2%)
EXTERNAL CIRCUIT
ISL6559 INTERNAL CIRCUIT
EN
+12V
VCC
+5V
POR
CIRCUIT
OV LATCH
SIGNAL
10.7k
1.40k
ENABLE
COMPARATOR
T
SS
2048
f
SW
-------------8.3ms
==
(EQ. 9)
FIGURE 8. RAMP CURRENT AND VOLTAGE FOR
REGULATING SOFT-START SLOPE
AND DURATION
-
+
IAVG
REFERENCE
EXTERNAL CIRCUIT
ISL6559 INTERNAL CIRCUIT
COMP
CC
RC
RFB
FB
IOUT
VDIFF
ERROR AMPLIFIER
VOLTAGE
VCOMP
VRAMP
IRAMP
IDEAL DIODES
ISL6559


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