The two most significant losses contributing to the
N-FET’s power dissipation are I
2
R losses and switching
losses. Select a transistor with low r
DS(ON)
and low
C
RSS
to minimize these losses.
Determine the maximum required gate-drive current
from the Q
g
specification in the N-FET data sheet.
The MAX773’s maximum allowed switching frequency
during normal operation is 300kHz; but at start-up the
maximum frequency can be 500kHz, so the maximum
current required to charge the N-FET’s gate is
f(max) x Q
g
(typ). Use the typical Q
g
number from the
transistor data sheet. For example, the Si9410DY has a
Q
g
(typ) of 17nC (at V
GS
= 5V), therefore the current
required to charge the gate is:
I
GATE
(max)
= (500kHz) (17nC) = 8.5mA.
The bypass capacitor on V+ (C2) must instantaneously
furnish the gate charge without excessive droop (e.g.,
less than 200mV):
Q
g
∆V+ = ——
C2
Continuing with the example, ∆V+ = 17nC/0.1µF = 170mV.
Use I
GATE
when calculating the appropriate shunt
resistor. See the
Shunt Regulator Operation
section.
Figure 2a’s application circuit uses an MTD3055EL
logic-level N-FET with a guaranteed threshold voltage
(V
TH
) of 2V. Figure 2b’s application circuit uses an
8-pin Si9410DY surface-mount N-FET that has 50mΩ
on resistance with 4.5V V
GS
, and a guaranteed V
TH
of
less than 3V.
NPN Transistors
The MAX773 can drive NPN transistors, but be
extremely careful when determining the base-current
requirements. Too little base current can cause exces-
sive power dissipation in the transistor; too much base
current can cause the base to oversaturate, so the tran-
sistor remains on continually. Both conditions can dam-
age the transistor.
When using the MAX773 with an NPN transistor, con-
nect EXTL to the transistor’s base, and connect R
BASE
between EXTH and the base (Figure 8c).
To determine the required peak inductor current,
I
C(PEAK
), observe the
Typical Operating Characteristics
efficiency graphs and the theoretical output current
capability vs. input voltage graphs to determine a
sense resistor that will allow the desired output current.
Divide the 170mV worst-case (smallest) voltage across
the current-sense amplifier V
CS
(max) by the sense-
resistor value. To determine I
B
, set the peak inductor
current (I
LIM)
equal to the peak transistor collector cur-
rent I
C(PEAK)
. Calculate I
B
as follows:
I
B
= I
LIM
/ß
Use the worst-case (lowest) value for ß given in the
transistor’s electrical specification, where the collector
current used for the test is approximately equal to I
LIM
.
It may be necessary to use even higher base currents
(e.g., I
B
= I
LIM
/10), although excessive I
B
may impair
operation by extending the transistor’s turn-off time.
R
BASE
is determined by:
(
V
EXTH
- V
BE
- V
CS
(min))
R
BASE
= ————————————–
I
B
Where V
EXTH
is the voltage at V+ (in bootstrapped
mode V
EXTH
is the output voltage), V
BE
is the 0.7V
transistor base-emitter voltage, V
CS
(min) is the voltage
drop across the current-sense resistor, and I
B
is the
minimum base current that forces the transistor into
saturation. This equation reduces to (V+ - 700mV -
170mV) / I
B
.
For maximum efficiency, make R
BASE
as large as pos-
sible, but small enough to ensure the transistor is
always driven near saturation. Highest efficiency is
obtained with a fast-switching NPN transistor
(f
T
≥ 150MHz) with a low collector-emitter saturation
voltage and a high current gain. A good transistor to
use is the Zetex ZTX694B.
Diode Selection
The MAX770–MAX773’s high switching frequency
demands a high-speed rectifier. Schottky diodes such
as the 1N5817–1N5822 are recommended. Make sure
that the Schottky diode’s average current rating
exceeds the peak current limit set by R
SENSE
, and that
its breakdown voltage exceeds V
OUT
. For high-temper-
ature applications, Schottky diodes may be inadequate
due to their high leakage currents; high-speed silicon
diodes may be used instead. At heavy loads and high
temperatures, the benefits of a Schottky diode’s low for-
ward voltage may outweigh the disadvantages of its
high leakage current.
Capacitor Selection
Output Filter Capacitor
The primary criterion for selecting the output filter
capacitor (C2) is low effective series resistance (ESR).
The product of the peak inductor current and the output
filter capacitor’s ESR determines the amplitude of the
ripple seen on the output voltage. An OS-CON 300µF,
6.3V output filter capacitor has approximately 50mΩ of
ESR and typically provides 180mV ripple when
stepping up from 3V to 5V at 1A (Figure 2a).
MAX770–MAX773
5V/12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controllers
______________________________________________________________________________________
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