MAX1584 Datasheet, PDF (22/29 Pages) Maxim Integrated Products – 5-Channel Slim DSC Power Supplies

5-Channel Slim DSC Power Supplies
Significant MOSFET selection parameters are as fol-
lows:
• On-resistance (R_DS(ON))
• Maximum drain-to-source voltage (V_DS(MAX))
• Total gate charge (Q_G)
• Reverse transfer capacitance (C_RSS)
DL1 and DL3 swing between PVSU and GND. DL2
swings between INDL2 and GND. Use a MOSFET with
on-resistance specified at or below the DL_ drive volt-
age. The gate charge, Q_G, includes all capacitance
associated with charging the gate and helps to predict
MOSFET transition time between on and off states.
MOSFET power dissipation is a combination of on-
resistance and transition losses. The on-resistance loss
is as follows:
P_RDSON = D x I_L² x R_DS(ON)
where D is the duty cycle, I_L is the average inductor
current, and R_DS(ON) is the MOSFET on-resistance. The
transition loss is approximately:
P_TRANS = (V_OUT x I_L x f_OSC x t_T) / 3
where V_OUT is the output voltage, I_L is the average
inductor current, f_OSC is the switching frequency, and
t_T is the transition time. The transition time is approxi-
mately Q_G / I_G , where Q_G is the total gate charge, and
I_G is the gate-drive current (0.5A typ). The total power
dissipation in the MOSFET is as follows:
P_MOSFET = P_RDSON + P_TRANS
Diode
For most AUX applications, a Schottky diode rectifies
the output voltage. Schottky low forward voltage and
fast recovery time provide the best performance in
most applications. Silicon signal diodes (such as
1N4148) are sometimes adequate in low-current
(<10mA), high-voltage (>10V) output circuits where the
output voltage is large compared to the diode forward
voltage.
AUX Compensation
The auxiliary controllers employ voltage-mode control
to regulate their output voltage. Optimum compensa-
tion depends on whether the design uses continuous or
discontinuous inductor current.
AUX Step-Up, Discontinuous Inductor Current
When the inductor current falls to zero on each switch-
ing cycle, it is described as discontinuous. The inductor
is not utilized as efficiently as with continuous current,
but in light-load applications, this often has little nega-
tive impact since the coil losses may already be low
compared to other losses. A benefit of discontinuous
inductor current is more flexible loop compensation, and
no maximum duty-cycle restriction on boost ratio.
To ensure discontinuous operation, the inductor must
have a sufficiently low inductance to fully discharge on
each cycle. This occurs when:
L < [V_IN² (V_OUT - V_IN) / V_OUT³] [R_LOAD / (2f_OSC)]
A discontinuous current boost has a single pole at the
following:
F_P = (2V_OUT - V_IN) / (2π x R_LOAD x C_OUT x V_OUT)
Choose the integrator cap so the unity-gain crossover,
f_C, occurs at f_OSC / 10 or lower. For many AUX circuits,
such as those powering motors, LEDs, or other loads
that do not require fast transient response, it is often
acceptable to overcompensate by setting f_C at f_OSC /
20 or lower.
C_C is then determined by the following:
C_C = [2V_OUT x V_IN / ((2V_OUT - V_IN) x V_RAMP)] [V_OUT /
(K(V_OUT - V_IN))]^1/2 [(V_FB / V_OUT)(g_M / (2π x f_C))]
where:
K = 2L x f_OSC / R_LOAD
and V_RAMP is the internal voltage ramp of 1.25V.
The C_C R_C zero is then used to cancel the f_P pole, so:
R_C = R_LOAD x C_OUT x V_OUT / [(2V_OUT - V_IN) x C_C]
AUX Step-Up, Continuous Inductor Current
Continuous inductor current can sometimes improve
boost efficiency by lowering the ratio between peak
inductor current and output current. It does this at the
expense of a larger inductance value that requires larg-
er size for a given current rating. With continuous
inductor-current boost operation, there is a right-half-
plane zero, Z_RHP, at the following:
Z_RHP = (1 - D)² R_LOAD / (2π x L)
where (1 - D) = V_IN / V_OUT (in a boost converter)
There is a complex pole pair at the following:
f₀ = V_OUT / [2π x V_IN (L x C_OUT)^1/2]
If the zero due to the output capacitor capacitance and
ESR is less than 1/10 the right-half-plane zero:
Z_COUT = 1 / (2π x C_OUT x R_ESR) < Z_RHP / 10
Then choose C_C so the crossover frequency f_C occurs
at Z_COUT. The ESR zero provides a phase boost at
crossover:
C_C = (V_IN / V_RAMP)(V_FB / V_OUT)(g_M / (2π x Z_COUT))
Choose R_C to place the integrator zero, 1 / (2π x R_C x
C_C), at f₀ to cancel one of the pole pairs:
R_C = V_IN (L x C_OUT)^1/2 / (V_OUT x C_C)
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