MAX1887EEE Maxim Integrated Products, MAX1887EEE Datasheet - Page 17

• Manufacturer Part Number: MAX1887EEE
• Description: Current Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1887EEE

• Number Of Outputs: 1
• Mounting Style: SMD/SMT
• Package / Case: QSOP-16
• Switching Frequency: 550 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Boost

Figure 3. “Valley” Current-Limit Threshold Point
There also is a negative current limit that prevents
excessive reverse inductor currents when V
ing current. The negative current-limit threshold is set to
approximately 150% of the positive current-limit thresh-
old, and tracks the positive current limit when ILIM is
adjusted.
The MAX1887/MAX1897 uses CS+ and CS- to differen-
tially measure the current across an external sense
resistor (R
put capacitors. This configuration provides precise cur-
rent balancing, current limiting, and voltage positioning
with a 1% current-sense resistor. Reducing the sense
voltage decreases power dissipation but increases the
relative measurement error.
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors don’t corrupt the current-
sense signals measured at CS+ and CS-. The IC should
be mounted relatively close to the current-sense resistor
with short, direct traces making a Kelvin sense connec-
tion.
The Quick-PWM controllers that may be used as the
master controller typically use the low-side MOSFET’s
on-resistance as its current-sense element. This depen-
dence on a loosely specified resistance with a large
temperature coefficient causes inaccurate current limit-
ing. As a result, high current-limit thresholds are need-
ed to guarantee full-load operation under worst-case
conditions. Furthermore, the inaccurate current limit
mandates the use of MOSFETs and inductors with
excessively high current and power dissipation ratings.
0
CS
Master Current-Limit Adjustment (LIMIT)
) connected between the inductor and out-
______________________________________________________________________________________
I
LIMIT(VALLEY)
TIME
= I
LOAD(MAX)
Quick-PWM Slave Controllers for
Multiphase, Step-Down Supplies
( )
2 - LIR
2η
OUT
I
I
I
PEAK
LOAD
LIMIT
is sink-
The slave includes a precision current-limit comparator
that supplements the master’s current-limit circuitry.
The MAX1887/MAX1897 uses CM+ and CM- to differ-
entially sense the master’s inductor current across a
current-sense resistor, providing a more accurate cur-
rent limit. When the master’s current-sense voltage
exceeds the current limit set by ILIM in the slave (see
the Dual-Mode ILIM Input section), the open-drain cur-
rent-limit comparator pulls LIMIT low (Figure 2). Once
the master triggers the current limit, a pulse-width mod-
ulated output signal appears at LIMIT. This signal is fil-
tered and used to adjust the master’s current-limit
threshold.
The gate drive voltage for the high-side, N-channel
MOSFET is generated by the flying capacitor boost cir-
cuit (Figure 4). The capacitor between BST and LX is
alternately charged from the external 5V bias supply
(V
gate-source terminals.
On startup, the synchronous rectifier (low-side MOSFET)
forces LX to ground and charges the boost capacitor to
5V. On the second half of each cycle, the switch-mode
power supply turns on the high-side MOSFET by closing
an internal switch between BST and DH. This provides
the necessary gate-to-source voltage to turn on the high-
side switch, an action that boosts the 5V gate drive signal
above the system’s main supply voltage (V+).
Figure 4. High-Side Gate Driver Boost Circuitry
DD
) and placed in parallel with the high-side MOSFET’s
MAX1887
MAX1897
High-Side, Gate Driver Supply (BST)
BST
DH
V+
LX
(R
BST
)*
( )* OPTIONAL–THE RESISTOR REDUCES
THE SWITCHING-NODE RISE TIME.
D
C
BST
C
BYP
BST
INPUT
(V
IN
N
)
H
L
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