Rainbow Electronics MAX8775 - Manual (Page 27)

where I LIMIT is the peak current allowed by the current-
limit circuit, including threshold tolerance and sense-
resistance variation. The MOSFETs must have a
relatively large heatsink to handle the overload power
dissipation.
Choose a Schottky diode (D
L
) with a forward voltage
drop low enough to prevent the low-side MOSFET’s
body diode from turning on during the dead time.
Boost Capacitors
The boost capacitors (C BST ) must be selected large
enough to handle the gate charging requirements of
the high-side MOSFETs. Typically, 0.1µF ceramic
capacitors work well for low-power applications driving
medium-sized MOSFETs. However, high-current appli-
cations driving large, high-side MOSFETs require boost
capacitors larger than 0.1µF. For these applications,
select the boost capacitors to avoid discharging the
capacitor more than 200mV while charging the high-
side MOSFETs’ gates:
where Q GATE is the total gate charge specified in the
high-side MOSFETs’ data sheet. For example, assume
the SI7634DP n-channel MOSFET is used on the high
side. According to the manufacturer’s data sheet, a sin-
gle SI7634DP has a gate charge of 21nC (V
GS
= 5V).
Using the above equation, the required boost capaci-
tance would be:
Selecting the closest standard value, this example
requires a 0.1µF ceramic capacitor.
Applications Information
Duty-Cycle Limits
Minimum Input Voltage
The minimum input operating voltage (dropout voltage)
is restricted by the maximum duty-cycle specification
(see the
Electrical Characteristics table). However,
keep in mind that the transient performance gets worse
as the step-down regulators approach the dropout volt-
age, so bulk output capacitance must be added (see
the voltage sag and soar equations in the
Design
Procedure section). The absolute point of dropout
occurs when the inductor current ramps down during
the off-time (ΔI
DOWN ) as much as it ramps up during
the on-time (ΔI UP ). This results in a minimum operating
voltage defined by the following equation:
where V
CHG
and V
DIS
are the parasitic voltage drops in
the charge and discharge paths, respectively. A rea-
sonable minimum value for h is 1.5, while the absolute
minimum input voltage is calculated with h = 1.
Maximum Input Voltage
The MAX8775 controller includes a minimum on-time
specification, which determines the maximum input
operating voltage that maintains the selected switching
frequency (see the
Electrical Characteristics table).
Operation above this maximum input voltage results in
pulse-skipping operation, regardless of the operating
mode selected by
SKIP_. At the beginning of each
cycle, if the output voltage is still above the feedback
threshold voltage, the controller does not trigger an on-
time pulse, effectively skipping a cycle. This allows the
controller to maintain regulation above the maximum
input voltage, but forces the controller to effectively
operate with a lower switching frequency. This results
in an input threshold voltage at which the controller
begins to skip pulses (V
IN(SKIP) ):
where f OSC is the switching frequency selected by OSC.
PCB Layout Guidelines
Careful PCB layout is critical to achieving low switching
losses and clean, stable operation. The switching
power stage requires particular attention (Figure 11). If
possible, mount all the power components on the top
side of the board, with their ground terminals flush
against one another. Follow these guidelines for good
PCB layout:
• Keep the high-current paths short, especially at the
ground terminals. This practice is essential for sta-
ble, jitter-free operation.
• Keep the power traces and load connections short.
This practice is essential for high efficiency. Using
thick copper PCB (2oz vs. 1oz) can enhance full-
load efficiency by 1% or more. Correctly routing
PCB traces is a difficult task that must be
approached in terms of fractions of centimeters,
V V
f t
IN SKIP OUT
OSC ON MIN
( )
( )
=
⎛
⎝⎜ ⎞
⎠⎟
1
V V V h
D
V V IN MIN OUT CHG
MAX
OUT DIS ( )
= + + −
⎛
⎝⎜ ⎞
⎠⎟
+
( ) 1
1
C
nC
mV
F
BST
= =
13
200
0 105 . μ
C
Q
mV
BST
GATE =
200
I I
I
LOAD LIMIT
INDUCTOR = − ⎛
⎝⎜
⎞
⎠⎟ Δ
2
MAX8775
Dual and Combinable Graphics Core
Controller for Notebook Computers
______________________________________________________________________________________ 27