MAX1887 Maxim Integrated Products, MAX1887 Datasheet - Page 12

• Manufacturer Part Number: MAX1887
• Description: (MAX1887 / MAX1897) Quick-PWM Slave Controllers
• Manufacturer: Maxim Integrated Products

transition on the TRIG input, the slave controller’s
inductor current is below its current-limit threshold, and
the minimum off time has expired. For the MAX1887, a
rising edge on the trigger input (TRIG) initiates a new
cycle. For the MAX1897, the trigger input’s polarity is
selected by connecting POL to V
GND (falling edge).
At the slave controller’s core is the one-shot that sets
the high-side switch’s on-time. This fast, low-jitter one-
shot adjusts the on-time in response to the input volt-
age and the difference between the inductor currents in
the master and the slave. Two identical transconduc-
tance amplifiers (G
between the master and slave current-sense signals.
The summed output is connected to COMP, allowing
adjustment of the integration time constant with a com-
pensation capacitor connected at COMP. The resulting
compensation current and voltage may be determined
by the following equations:
where Z
The PWM controller uses this integrated signal (V
to set the slave controller’s on time. When the master
and slave current-sense signals (CM+ to CM- and CS+
Quick-PWM Slave Controllers for
Multiphase, Step-Down Supplies
12
Table 2. Component Suppliers
MOSFETS
Fairchild Semiconductor
International Rectifier
Siliconix
CAPACITORS
Kemet
Panasonic
Sanyo
Taiyo Yuden
INDUCTORS
Coilcraft
Coiltronics
Sumida
I
V
COMP
______________________________________________________________________________________
COMP
COMP
MANUFACTURER
=
=
G
V
MM
is the impedance at the COMP output.
OUT
(
V
+
MM
CM
I
COMP COMP
+
= G
−
V
CM
Z
MS
−
) integrate the difference
)
−
G
CC
MS
(rising edge) or to
(
V
CS
+
−
V
[65] 281-3226 (Singapore)
CS
[03] 3667-3408 (Japan)
[COUNTRY CODE]
−
[1] 888-522-5372
[1] 310-322-3331
[1] 203-268-6261
[1] 408-986-0424
[1] 847-468-5624
[1] 408-749-9714
[1] 408-573-4150
[1] 800-322-2645
[1] 561-752-5000
[1] 408-982-9660
COMP
)
PHONE
)
to CS-) become unbalanced, the transconductance
amplifiers adjust the slave controller’s on time, allowing
the slave inductor current to increase or decrease until
the current-sense signals are properly balanced.
This control algorithm results in balanced inductor cur-
rents with the slave switching frequency synchronized
to the master. Since the master operates at nearly con-
stant frequency, the slave will as well. The benefits of a
constant switching frequency are twofold: first, the fre-
quency can be selected to avoid noise-sensitive
regions of the spectrum; second, the inductor ripple-
current operating point remains relatively constant,
resulting in easy design methodology and predictable
output voltage ripple.
Multiple phase switching effectively distributes the load
among the external components, thereby improving the
overall efficiency. Distributing the load current between
multiple phases lowers the peak inductor current by the
t
ON
= (Master’s on time) + (Slave’s on-time
=
=
correction due to current imbalance)
K
K
www.fairchildsemi.com
www.irf.com
www.vishay.com
www.kemet.com
www.panasonic.com
www.secc.co.jp
www.t-yuden.com
www.coilcraft.com
www.coiltronics.com
www.sumida.com






V
V
V
COMP
OUT
V
IN
IN

 +




K



WEBSITE
I
COMP C
V
IN
Z


