ISL6219A_07 Datasheet, PDF (8/17 Pages) Intersil Corporation – Microprocessor CORE Voltage Regulator Precision Multi-Phase BUCK PWM Controller for Mobile Applications

ISL6219A
The converter depicted in Figure 3 delivers 36A to a 1.5V
load from a 12V input. The RMS input capacitor current is
5.9A. Compare this to a single-phase converter also down
12V to 1.5V at 36A. The single-phase converter has 11.9A
rms input capacitor current. The single-phase converter
must use an input capacitor bank with twice the RMS current
capacity as the equivalent three-phase converter.
Figures 14 and 15 in the section entitled can be used to
determine the input-capacitor rms current based on load
current, duty cycle, and the number of active channels. They
are provided as aids in determining the optimal input
capacitor solution. Figure 16 shows the single phase input-
capacitor rms current for comparisson.
PWM OPERATION
The number of active channels selected determines the
timing for each channel. By default, the timing mode for the
ISL6219A is 3-phase. The designer can select 2-phase
timing by connecting PWM3 to VCC.
One switching cycle for the ISL6219A is defined as the time
between PWM1 pulse termination signals (the internal signal
that initiates a falling edge on PWM1). The cycle time is the
inverse of the switching frequency selected by the resistor
connected between the FS/EN pin and ground (see
Switching Frequency). Each cycle begins when a clock signal
commands the channel-1 PWM output to go low. This
signals the channel-1 MOSFET driver to turn off the channel-
1 upper MOSFET and turn on the channel-1 synchronous
MOSFET. If two-channel operation is selected, the PWM2
pulse terminates 1/2 of a cycle later. If three channels are
selected the PWM2 pulse terminates 1/3 of a cycle after
PWM1, and the PWM3 output will follow after another 1/3 of
a cycle.
Once a channel’s PWM pulse terminates, it remains low for
a minimum of 1/4 cycle. This forced off time is required to
assure an accurate current sample as described in Current
Sensing. Following the 1/4-cycle forced off time, the
controller enables the PWM output. Once enabled, the PWM
output transitions high when the sawtooth signal crosses the
adjusted error-amplifier output signal, V_COMP as illustrated
in Figures 1 and 5. This is the signal for the MOSFET driver
to turn off the synchronous MOSFET and turn on the upper
MOSFET. The output will remain high until the clock signals
the beginning of the next cycle by commanding the PWM
pulse to terminate.
CURRENT SENSING
Intersil multi-phase controllers sense current by sampling the
voltage across the lower MOSFET during its conduction
interval. MOSFET r_DS(ON) sensing is a no-added-cost
method to sense current for load-line regulation, channel-
current balance, module current sharing, and overcurrent
protection. If desired, an independent current-sense resistor
in series with the lower-MOSFET source can serve as a
sense element in place of the MOSFET r_DS(ON).
I_n
ISEN
=
IL
-^r--^D-----^S----⁽---^O-----^N-----⁾
RISEN
V_IN
CHANNEL N
UPPER MOSFET
SAMPLE
&
HOLD
-
+
I_L
ISEN(n)
R_ISEN
-
IL rDS_(ON)
+
CHANNEL N
LOWER MOSFET
ISL6219A INTERNAL CIRCUIT EXTERNAL CIRCUIT
FIGURE 4. INTERNAL AND EXTERNAL CURRENT-SENSING
CIRCUITRY
The ISEN input for each channel uses a ground-referenced
amplifier to reproduce a signal proportional to the channel
current (Figure 4). After sufficient settling time, the sensed
current is sampled, and the sample is used for current
balance, load-line regulation and overcurrent protection. The
ISL6219A samples channel current once each cycle.
Figure 4 shows how the sampled current, I_n, is created from
the channel current I_L. The circuitry in Figure 4 represents
the current measurement and sampling circuitry for channel
n in an N-channel converter. This circuitry is repeated for
each channel in the converter but will not be active in
unused channels.
CHANNEL-CURRENT BALANCE
Another benefit of multi-phase operation is the thermal
advantage gained by distributing the dissipated heat over
multiple devices and greater area. By doing this, the
designer avoids the complexity of driving multiple parallel
MOSFETs and the expense of using expensive heat sinks
and exotic magnetic materials.
In order to fully realize the thermal advantage, it is important
that each channel in a multi-phase converter be controlled to
deliver about the same current at any load level. Intersil
multi-phase controllers guarantee current balance by
comparing each channel’s current to the average current
delivered by all channels and making an appropriate
adjustment to each channel’s pulse width based on the error.
Intersil’s patented current-balance method is illustrated in
Figure 5 where the average of the 2 or 3 sampled channel
currents combines with the channel 1 sample, I₁, to create
an error signal I_ER. The filtered error signal modifies the
pulse width commanded by V_COMP to correct any
unbalance and force I_ER toward zero.
8
FN9093.1
March 20, 2007