Rainbow Electronics MAX8720 - Manual (Page 27)

where N is the number of high-side MOSFETs used for
one regulator, and Q
GATE is the gate charge specified
in the MOSFET’s data sheet. For example, assume the
IRF7821 n-channel MOSFET is used on the high side.
According to the manufacturer’s data sheet, a single
IRF7821 has a maximum gate charge of 14nC (V
GS =
5V). Using the above equation, the required boost
capacitance is:
Select the closest standard value. This example
requires a 0.1µF ceramic capacitor.
Applications Information
Dropout Performance
The output-voltage adjust range for continuous-conduc-
tion operation is restricted by the nonadjustable 500ns
(max) minimum off-time one-shot (375ns max at
1000kHz). For best dropout performance, use the slower
(200kHz) on-time settings. When working with low input
voltages, the duty-factor limit must be calculated using
worst-case values for on- and off-times. Manufacturing
tolerances and internal propagation delays introduce an
error to the TON K-factor. This error is greater at higher
frequencies (Table 3). Also, keep in mind that transient-
response performance of buck regulators operated
close to dropout is poor, and bulk output capacitance
must often be added (see the V
SAG equation in the
Design Procedure section).
The absolute point of dropout is when the inductor cur-
rent ramps down during the minimum off-time (
∆I
DOWN )
as much as it ramps up during the on-time ( ∆I
UP
). The
ratio h = ∆I
UP
/ ∆I
DOWN
is an indicator of the ability to
slew the inductor current higher in response to
increased load, and must always be greater than 1. As
h approaches 1, the absolute minimum dropout point,
the inductor current is less able to increase during
each switching cycle and V
SAG
greatly increases
unless additional output capacitance is used.
A reasonable minimum value for h is 1.5, but this may
be adjusted up or down to allow tradeoffs between
V
SAG
, output capacitance, and minimum operating
voltage. For a given value of h, the minimum operating
voltage can be calculated as:
where V DIS and V CHG are the parasitic voltage drops in
the discharge and charge paths, respectively (see the
On-Time One-Shot (TON) section), t
OFF(MIN) is from the
Electrical Characteristics table, and K is taken from
Table 3. The absolute minimum input voltage is calcu-
lated with h = 1.
If the calculated V IN(MIN) is greater than the required
minimum input voltage, then operating frequency must
be reduced or output capacitance added to obtain
an acceptable V
SAG . If operation near dropout is
anticipated, calculate V SAG to be sure of adequate
transient response.
Dropout Design Example:
V OUT = 1.6V
f SW = 550kHz
K = 1.8µs, worst-case K = 1.58µs
t OFF(MIN) = 500ns
V DIS = V CHG = 100mV
h = 1.5
V
IN(MIN)
= (1.6V + 0.1V) / (1 - 0.5µs x 1.5 / 1.58µs)
+ 0.1V - 0.1V = 3.2V
Calculating again with h = 1 gives the absolute limit
of dropout:
V IN(MIN) = (1.6V + 0.1V) / (1 - 0.5µs x 1.0 / 1.58µs)
+ 0.1V - 0.1V = 2.5V
Therefore, V IN must be greater than 2.5V, even with
very large output capacitance, and a practical input
voltage with reasonable output capacitance is 3.2V.
One-Stage (Battery Input) vs. Two-Stage
(5V Input) Applications
The MAX8720 can be used with a direct battery con-
nection (one stage) or can obtain power from a regulat-
ed 5V supply (two stage). Each approach has
advantages, and careful consideration should go into
the selection of the final design.
The one-stage approach offers smaller total inductor
size and fewer capacitors overall due to the reduced
demands on the 5V supply. The transient response of
the single stage is better due to the ability to ramp up
the inductor current faster. The total efficiency of a sin-
gle stage is better than the two-stage approach.
The two-stage approach allows flexible placement due
to smaller circuit size and reduced local power dissipa-
tion. The power supply can be placed closer to the
CPU for better regulation and lower I
2 R losses from PC
board traces. Although the two-stage design has worse
transient response than the single stage, this can be
offset by the use of a voltage-positioned converter.
V
V V
t h
K
V V
IN MIN
OUT DIS
OFF MIN
CHG DIS ( )
( )
=
+
−
× 
 

+ −
1
C
nC
mV
F
BST
= × =
1 14
200
0 07 . µ
MAX8720
Dynamically Adjustable 6-Bit VID
Step-Down Controller
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