LM3477_14 Datasheet, PDF (20/34 Pages) Texas Instruments – High Efficiency High-Side N-Channel Controller for Switching Regulator

LM3477
SNVS141J – MAY 2004 – REVISED JULY 2004
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ΔV_c = ΔV_r + ΔV_q
(24)
The design objective is to keep ΔV_c lower than some maximum overshoot (V_OS(MAX)). V_OS(MAX) is chosen based
on the output load requirements.
Both voltages ΔV_r and ΔV_qwill change with time. For ΔV_r the equation is:
'V_r = R_ESR('I_OUT(MAX) -
V_{OUT -} D_MINV_IN
L
t) (V)
(25)
where,
R_ESR = the output capacitor ESR
ΔI_OUT = the difference between the load current change I_OUT(MAX) − I_OUT(MIN)
D_MIN = Minimum duty cycle of device (0.165 typical)
Evaluating this equation at t = 0 gives ΔV_r(max). Substituting V_OS(MAX) for ΔV_r(MAX) and solving for R_ESR gives:
V_OS(MAX)
R_ESR(MAX) = _'IOUT(MAX) :
(26)
The expression for ΔV_q is:
'V_q =
'I_OUT(MAX)
C_OUT
t
V_{OUT -} D_MINV_IN
-
2 X L X C_OUT
t² (V)
(27)
From Figure 14 it can be told that ΔV_C will reach its peak value at some point in time and then decrease. The
larger the output capacitance is, the earlier the peak will occur. To find the peak position, let the derivative of ΔV_C
go to zero, and the result is:
tpeak = ^{'IOUT(MAX) x L} _{- COUTRESR}
V_OUT-D_MINV_IN
(28)
'V_c
'V_q
'V_r
0
t_peak
Time
Figure 14. Output Voltage Overshoot Peak
The intention is to find the capacitance value that will yield, at t_peak, a ΔV_C that equals V_OS(max). Substituting tpeak
for t and equating ΔV_C to V_OS(max) gives the following solution for C_OUT(MIN):
L ^VOS(MAX)^-
C_OUT(MIN)=
V_OS(MAX)2 - ('I_OUT(MAX) x R_ESR)²
(F)
(V_OUT) R_ESR2
(29)
The chosen output capacitance should not be less than 47µF, even if the solution for C_OUT(MIN) is less than 47µF.
Notice it is already assumed that the total ESR is no greater than R_ESR(MAX), otherwise the term under the square
root will be a negative number.
Power Mosfet Selection
The drive pin of LM3477/A must be connected to the gate of an external MOSFET. In a buck topology, the drain
of the external N-Channel MOSFET is connected to the input and the source is connected to the inductor. The
C_B pin voltage provides the gate drive needed for an external N-Channel MOSFET. The gate drive voltage
depends on the input voltage (see TYPICAL PERFORMANCE CHARACTERISTICS). In most applications, a
logic level MOSFET can be used. For very low input voltages, a sub-logic level MOSFET should be used.
The selected MOSFET directly controls the efficiency. The critical parameters for selection of a MOSFET are:
1. Minimum threshold voltage, V_TH(MIN)
20
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