Rainbow Electronics MAX1636 - Manual (Page 20)

MAX1636
Low-Voltage, Precision Step-Down
Controller for Portable CPU Power
20 ______________________________________________________________________________________
that both MOSFETs are within their maximum junction
temperature at high ambient temperature. The worst-
case dissipation for the high-side MOSFET occurs at
both extremes of input voltage, and the worst-case dis-
sipation for the low-side MOSFET occurs at maximum
input voltage.
Duty = (V OUT + V Q2 ) / (V IN - V Q1 )
PD (upper FET) = I LOAD
2 x R
DS(ON) x Duty + V IN x
I
LOAD
x f x [(V
IN
x C
RSS
) / I
GATE
+ 20ns]
PD (lower FET) = I LOAD
2 x R
DS(ON) x (1 - Duty)
where on-state voltage drop V
Q
= I
LOAD
x R
DS(ON)
,
C RSS = MOSFET reverse transfer capacitance, I GATE =
DH driver peak output current capability (1A typ), and
20ns = DH driver inherent rise/fall time. The MAX1636’s
output undervoltage shutdown protects the synchro-
nous rectifier under output short-circuit conditions. To
reduce EMI, add a 0.1µF ceramic capacitor from the
high-side switch drain to the low-side switch source.
Rectifier Clamp Diode
The rectifier is a clamp across the low-side MOSFET
that catches the negative inductor swing during the
60ns dead time between turning one MOSFET off and
each low-side MOSFET on. The latest generations of
MOSFETs incorporate a high-speed silicon body
diode, which serves as an adequate clamp diode if
efficiency is not of primary importance. A Schottky
diode can be placed in parallel with the body diode to
reduce the forward voltage drop, typically improving
efficiency 1% to 2%. Use a diode with a DC current rat-
ing equal to one-third of the load current; for example,
use an MBR0530 (500mA-rated) type for loads up to
1.5A, a 1N5819 type for loads up to 3A, or a 1N5822
type for loads up to 10A. The rectifier’s rated reverse-
breakdown voltage must be at least equal to the maxi-
mum input voltage, preferably with a 20% derating
factor.
Boost-Supply Diode
A signal diode such as a 1N4148 works well in most
applications. If the input voltage can go below +6V,
use a small (20mA) Schottky diode for slightly
improved efficiency and dropout characteristics. Do
not use large power diodes, such as 1N5817 or
1N4001, since high junction capacitance can pump up
VL to excessive voltages.
Low-Voltage Operation
Low input voltages and low input-output differential
voltages each require extra care in their design. Low
absolute input voltages can cause the VL linear regula-
tor to enter dropout and eventually shut itself off. Low
V
IN
- V
OUT
differentials can cause the output voltage to
sag when the load current changes abruptly. The sag’s
amplitude is a function of inductor value and maximum
duty factor (D
MAX , an Electrical Characteristics
parameter, 98% guaranteed over temperature at f =
200kHz) as follows:
Table 6 is a low-voltage troubleshooting guide. The
cure for low-voltage sag is to increase the output
capacitor’s value. For example, at V
IN = +5.5V, V OUT =
5V, L = 10µH, f = 200kHz, and I STEP = 3A, a total
capacitance of 660µF keeps the sag less than 200mV.
Note that only the capacitance requirement increases;
the ESR requirements do not change. Therefore, the
added capacitance can be supplied by a low-cost bulk
capacitor in parallel with the normal low-ESR capacitor.
__________Applications Information
Heavy-Load Efficiency Considerations
The major efficiency-loss mechanisms under loads are
as follows, in the usual order of importance:
• P(I 2 R) = I 2 R losses
• P(tran) = transition losses
• P(gate) = gate-charge losses
• P(diode) = diode-conduction losses
• P(cap) = capacitor ESR losses
• P(IC) = losses due to the IC’s operating supply
current
Inductor core losses are fairly low at heavy loads
because the inductor’s AC current component is small.
Therefore, they are not accounted for in this analysis.
Ferrite cores are preferred, especially at 300kHz, but
powdered cores, such as Kool-Mu, can also work well.
Efficiency = P
OUT
/ P
IN
x 100%
= P OUT / (P OUT + P TOTAL ) x 100%
P TOTAL = P(I 2 R) + P(tran) + P(gate) + P(diode)
+ P(cap) + P(IC)
P = (I 2 R) = (I LOAD )2 x (R DC + R DS(ON) +R SENSE )
V
I x L
x C V x D V
SAG
STEP
F IN MIN MAX OUT
( )
( )
( )
=
Ч −
2
2