Datasheet

(4) Gate threshold voltage (V
TH(MIN)
)
(5) Turn-on/turn-off times
(6) Turn-on/turn-off delays
At high switching rates, dynamic characteristics (para-
meters 1, 2, 5, and 6) that predict switching losses may
have more impact on efficiency than R
DS(ON)
, which pre-
dicts DC losses. Q
G
includes all capacitance associated
with charging the gate, and best performance is
achieved with a low total gate charge. Q
G
also helps
predict the current needed to drive the gate at the
selected operating frequency. This is very important
because the output current from the charge pump is
finite (50mA, max) and is used to drive the gates of the
MOSFETs as well as provide bias for the IC. R
DS(ON)
is
important as well, as it is used for current sensing in the
MAX1960/MAX1961. R
DS(ON)
also causes power dissi-
pation during the on-time of the MOSFET.
Choose Q
G
to be as low as possible. Ensure that:
Choose R
DS(ON)
to provide the desired I
LOAD(MAX)
at
the desired current-limit threshold voltage (see the
Setting the Current Limit section).
MOSFET RC Snubber Circuit
Fast-switching transitions can cause ringing due to res-
onating circuit parasitic inductance and capacitance at
the switching nodes. This high-frequency ringing
occurs at LX rising and falling transitions, and may
introduce current-sensing errors and generate EMI. To
dampen this ringing, a series RC snubber circuit can
be added across each MOSFET switch (Figure 8).
Typical values for the snubber components are C
SNUB
= 4700pF and R
SNUB
= 1Ω, however, the ideal values
for snubber components will depend on circuit para-
sitics. Below is the procedure for selecting the compo-
nent values of the series RC snubber circuit:
1) Connect a scope probe to measure V
LX
to GND,
and observe the ringing frequency, f
R
.
2) Find the capacitor value (connected from LX to
GND) that reduces the ringing frequency by half.
3) The circuit parasitic capacitance, C
PAR
, at LX is then
equal to 1/3 of the value of the added capacitance
above.
QQ
mA
f
GG
S
12
50
+≤
MAX1960/MAX1961/MAX1962
2.35V to 5.5V, 0.5% Accurate, 1MHz PWM
Step-Down Controllers with Voltage Margining
______________________________________________________________________________________ 19
FEEDBACK DIVIDER
ERROR AMPLIFIER
V
1
R1
R2 R3
C9
R
S
L1
V
2
R
ESR
C
OUT
R
LOAD
0.8V
MODULATOR OUTPUT FILTER
Gm
V
IN
/V
RAMP
Figure 7. Open-Loop Transfer Model