MAX1652 Datasheet, PDF (22/28 Pages) Maxim Integrated Products – High-Efficiency, PWM, Step-Down DC-DC Controllers in 16-Pin QSOP

High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
Power from the main and secondary outputs is lumped
together to obtain an equivalent current referred to the
main output voltage (see Inductor Value section for def-
initions of parameters). Set the value of the current-
sense resistor at 80mV / I_TOTAL.
P_TOTAL = the sum of the output power from
all outputs
I_TOTAL = P_TOTAL / V_OUT = the equivalent output
current referred to V_OUT
V_OUT (V_IN(MAX) - V_OUT)
L(primary) = —————————————
V_IN(MAX) x f x I_TOTAL x LIR
V_SEC + V_FWD
Turns Ratio N = ——————————————
V_OUT(MIN) + V_RECT + V_SENSE
where: V_SEC is the minimum required rectified
secondary-output voltage
V_FWD is the forward drop across the
secondary rectifier
V_OUT(MIN) is the minimum value of the main
output voltage (from the Electrical
Characteristics)
V_RECT is the on-state voltage drop across the
synchronous-rectifier MOSFET
V_SENSE is the voltage drop across the sense
resistor
In positive-output (MAX1652) applications, the trans-
former secondary return is often referred to the main
output voltage rather than to ground in order to reduce
the needed turns ratio. In this case, the main output
voltage must first be subtracted from the secondary
voltage to obtain V_SEC.
______Selecting Other Components
MOSFET Switches
The two high-current N-channel MOSFETs must be
logic-level types with guaranteed on-resistance specifi-
cations at V_GS = 4.5V. Lower gate threshold specs are
better (i.e., 2V max rather than 3V max). Drain-source
breakdown voltage ratings must at least equal the max-
imum input voltage, preferably with a 20% derating
factor. The best MOSFETs will have the lowest on-resis-
tance per nanocoulomb of gate charge. Multiplying
R_DS(ON) x Q_G provides a meaningful figure by which to
compare various MOSFETs. Newer MOSFET process
technologies with dense cell structures generally give
the best performance. The internal gate drivers can tol-
erate more than 100nC total gate charge, but 70nC is a
more practical upper limit to maintain best switching
times.
In high-current applications, MOSFET package power
dissipation often becomes a dominant design factor.
I²R losses are distributed between Q1 and Q2 accord-
ing to duty factor (see the equations below). Switching
losses affect the upper MOSFET only, since the
Schottky rectifier clamps the switching node before the
synchronous rectifier turns on. Gate-charge losses are
dissipated by the driver and don’t heat the MOSFET.
Ensure that both MOSFETs are within their maximum
junction temperature at high ambient temperature by
calculating the temperature rise according to package
thermal-resistance specifications. The worst-case dissi-
pation for the high-side MOSFET occurs at the minimum
battery voltage, and the worst-case for the low-side
MOSFET occurs at the maximum battery voltage.
PD (upper FET) = I_LOAD2 x R_DS(ON) x DUTY
( ) + V_IN x I_LOAD x f x –V––^IN_I–G–x_A–_T–C_E–^R–^S–^S– +20ns
PD (lower FET) = I_LOAD2 x R_DS(ON) x (1 - DUTY)
DUTY = (V_OUT + V_Q2) / (V_IN - V_Q1 + V_Q2)
where the 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 typically)
20ns = DH driver inherent rise/fall time
Under output short circuit, the synchronous-rectifier
MOSFET suffers extra stress and may need to be over-
sized if a continuous DC short circuit must be tolerated.
During short circuit, Q2’s duty factor can increase to
greater than 0.9 according to:
Q2 DUTY (short circuit) = 1 - [V_Q2 / (V_IN(MAX) - V_{Q1 +} V_Q2)]
where the on-state voltage drop V_Q = (120mV / R_SENSE)
x R_DS(ON).
Rectifier Diode D1
Rectifier D1 is a clamp that catches the negative induc-
tor swing during the 60ns dead time between turning
off the high-side MOSFET and turning on the low-side.
D1 must be a Schottky type in order to prevent the
lossy parasitic MOSFET body diode from conducting. It
is acceptable to omit D1 and let the body diode clamp
the negative inductor swing, but efficiency will drop one
or two percent as a result. 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.
D1’s rated reverse breakdown voltage must be at least
equal to the maximum input voltage, preferably with a
20% derating factor.
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