Rainbow Electronics MAX1844 - Manual (Page 19)

pation limits often limits how small the MOSFET can be.
Again, the optimum occurs when the switching (AC)
losses equal the conduction (R
DS(ON) ) losses. High-side
switching losses do not usually become an issue until
the input is greater than approximately 15V.
Switching losses in the high-side MOSFET can become
an insidious heat problem when maximum AC adapter
voltages are applied, due to the squared term in the
CV
2 f switching loss equation. If the high-side MOSFET
chosen for adequate R DS(ON) at low battery voltages
becomes extraordinarily hot when subjected to
V
IN(MAX) , reconsider the choice of MOSFET.
Calculating the power dissipation in Q1 due to switching
losses is difficult, since it must allow for difficult-to-quanti-
fy factors that influence the turn-on and turn-off times.
These factors include the internal gate resistance, gate
charge, threshold voltage, source inductance, and PC
board layout characteristics. The following switching loss
calculation provides only a very rough estimate and is no
substitute for breadboard evaluation, preferably including
a sanity check using a thermocouple mounted on Q1.
where C RSS is the reverse transfer capacitance of Q1,
and I
GATE
is the peak gate-drive source/sink current (1A
typ).
For the low-side MOSFET, Q2, the worst-case power dis-
sipation always occurs at maximum battery voltage:
PD(Q2) = (1 - V OUT / V IN(MAX) ) ✕ I LOAD2 ✕ R DS(ON)
The absolute worst case for MOSFET power dissipation
occurs under heavy overloads that are greater than
I
LOAD(MAX) but are not quite high enough to exceed the
current limit. To protect against this possibility, you must
“overdesign” the circuit to tolerate I
LOAD = I LIMIT(HIGH) +
[(LIR / 2) ✕ I
LOAD(MAX)
], where I
LIMIT(HIGH)
is the maxi-
mum valley current allowed by the current-limit circuit,
including threshold tolerance and sense-resistance vari-
ation. If short-circuit protection without overload protec-
tion is adequate, enable undervoltage protection, and
use I
LOAD(MAX ) to calculate component stresses.
Choose a Schottky diode D1 having a forward voltage
drop low enough to prevent the Q2 MOSFET body diode
from turning on during the dead time. As a general rule,
a diode having a DC current rating equal to 1/3 of the
load current is sufficient. This diode is optional, and if
efficiency isn’t critical it can be removed.
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. For best dropout per-
formance, 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
5). Also, keep in mind that transient response perfor-
mance of buck regulators operated close to dropout is
poor, and bulk output capacitance must often be
added (see the V
SAG equation in the Transient
Response 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 indicates the circuit’s 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 will be less able to increase during
each switching cycle, and V
SAG will greatly increase
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 trade-offs 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
DROP1
and V
DROP2
are the parasitic voltage
drops in the discharge and charge paths, t OFF(MIN) is
from the Electrical Characteristics table, and K is taken-
from Table 5. The absolute minimum input voltage is cal-
culated 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 anticipat-
ed, calculate V SAG to be sure of adequate transient
response.
V
V V
t h
K
V V
IN MIN
OUT DROP
OFF MIN
DROP DROP ( )
( )
=
+
( )
×






+
1
2 1
1-
-
PD(Q1 switching)
C V f I
I
RSS IN(MAX)
2
LOAD
GATE
=
Ч Ч Ч
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook Computers
______________________________________________________________________________________ 19