ISL6553 View Datasheet(PDF) - Renesas Electronics
Part Name
Description
Manufacturer
ISL6553 Datasheet PDF : 15 Pages
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ISL6553
RIN
RFB Cc
FB
COMP
ISL6553
ERROR
AMPLIFIER
-
+
SAWTOOTH
GENERATOR
CORRECTION
+
-
COMPARATOR
-
+
PWM
CIRCUIT
REFERENCE
DAC
TO OTHER
CHANNELS
DIFFERENCE
+
-
AVERAGING
CURRENT
SENSING
CURRENT
SENSING
FROM
OTHER
CHANNEL
TO OVER
+
CURRENT
TRIP
-
COMPARATOR
REFERENCE
PWM
ISEN
VIN
Q1 L
HIP6601
Q2
IL
PHASE
RISEN
ONLY ONE OUTPUT
STAGE SHOWN
INDUCTOR
CURRENT
FROM
OTHER
CHANNEL
VCORE
FIGURE 7. SIMPLIFIED FUNCTIONAL BLOCK DIAGRAM SHOWING CURRENT AND VOLTAGE SAMPLING
Current Sensing and Balancing
Overview
The ISL6553 samples the on-state voltage drop across each
synchronous rectifier FET, Q2, as an indication of the inductor
current in that phase, see Figure 7. Neglecting AC effects (to
be discussed later), the voltage drop across Q2 is simply
rDS(ON)(Q2) x inductor current (IL). Note that IL, the inductor
current, is 1/2 of the total current (ILT).
The voltage at Q2’s drain, the PHASE node, is applied to the
RISEN resistor to develop the IISEN current to the ISL6553
ISEN pin. This pin is held at virtual ground, so the current
through RISEN is IL x rDS(ON)(Q2) / RISEN.
The IISEN current provides information to perform the following
functions:
1. Detection of an over-current condition
2. Reduce the regulator output voltage with increasing load
current (droop)
3. Balance the IL currents in the two phases
Over-Current, Selecting RISEN
The current detected through the RISEN resistor is averaged
with the current detected in the other channel. The averaged
current is compared with a trimmed, internally generated
current, and used to detect an over-current condition.
The nominal current through the RISEN resistor should be
50A at full output load current, and the nominal trip point for
over-current detection is 165% of that value, or 82.5A.
Therefore, RISEN = IL x rDS(ON) (Q2) / 50A.
For a full load of 25A per phase, and an rDS(ON) (Q2) of 4m,
RISEN = 2k.
The over-current trip point would be 165% of 25A, or ~ 41A per
phase. The RISEN value can be adjusted to change the over-
current trip point, but it is suggested to stay within 25% of
nominal.
Droop, Selection of RIN
The average of the currents detected through the RISEN
resistors is also steered to the FB pin. There is no DC return
path connected to the FB pin except for RIN, so the average
current creates a voltage drop across RIN. This drop increases
the apparent VCORE voltage with increasing load current,
causing the system to decrease VCORE to maintain balance at
the FB pin. This is the desired “droop” voltage used to maintain
VCORE within limits under transient conditions.
With a high dv/dt load transient, typical of high performance
microprocessors, the largest deviations in output voltage occur
FN4931 Rev 1.00
August 2004
Page 10 of 15
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