HV9606 Supertex, Inc., HV9606 Datasheet - Page 5

• Manufacturer Part Number: HV9606
• Description: Switchmode PWM Controller
• Manufacturer: Supertex, Inc.

Functional Block Diagram
Functional Description
The HV9606 is made up of several functional blocks. The
operation of each of these blocks is described in the following
sections.
Programmable Start/Stop Control Circuit
In applications such as Powered Ethernet and Pair Gain, power
is fed from a remote source over data-carrying wire pairs. For
various reasons each of these systems require that below some
voltage level on the line, the load shall appear as a very high
impedance. In all of these systems the leakage current at some
voltage level is specified. Start control provides this function by
holding the circuit in a high input impedance state until the input
voltage exceeds the programmed START threshold.
Since these systems provide power feed over the data wire
pairs, the input supply source impedance to the DC/DC con-
verter can be significant, from tens to hundreds of ohms. This
creates a significant problem since the open circuit voltage may
be sufficient to exceed the programmed START voltage, but as
soon as the DC/DC converter begins to draw current, the voltage
drop due to the resistance of the wire pairs significantly reduces
the effective voltage.
Let us consider a specific application situation where the maxi-
mum loop resistance of the wire pair is 20Ω, the minimum open
circuit voltage is 40V, the maximum open circuit voltage is 60V,
the maximum allowable current is 500mA and the minimum
current is 5mA. From these conditions it is evident that the
START
STOP
Vin
RT
Oscillator
SYNC
Programable
Start/Stop
Circuit
STATUS
CLK
D
Supervisor
Circuit
CLR
___
Vdd
Q
_
Q
SENSE
Regulator
Enable
Oscillator
Enable
Soft
Start
Enable
SS
Start-Up
Regulator
Generator
Bandgap
Reference
Circuit
VREF
Soft
Start
NI
FB
Vdd
UVLO
A
COMP
5
C
effective voltage range at the input of the DC/DC converter is 30V
to 59.9V. The minimum power available at the input of the DC/
DC converter is 30V x 500mA or 15W. Therefore, if the DC/DC
converter is required to deliver a guaranteed maximum 12.5W at
its output then its efficiency must be at least 84%.
Based on the above application conditions it is evident that the
DC/DC converter cannot be allowed to start until the effective
voltage exceeds 40V and must stop when the effective voltage
drops below 30V. If we impose an additional constraint that at
20V open circuit voltage on the wire pair, the resistor divider
cannot draw more than 1µA the values for the R1, R2 and R3
resistor divider can be calculated as follows, since the input
current on the start and stop pins is negligible.
V / I = 20V / 1µA = R1 + R2 + R3 = 20MΩ
V
R3 = 20MΩ x 7V / 40V
R3 = 3.5MΩ
V
V
R2 = (20MΩ x 7V / 30V) – 3.5MΩ
R2 = 1.17MΩ
And
START
STOP
STOP
R
R
/ V
/ V
C
/ V
VX2
UVLO
IN
IN
IN
= 7V / 30V = (R2 +R3) / (R1 + R2 + R3
= (R2 + 3.5MΩ) / 20MΩ
= 7V / 40V = R3 / (R1 + R2 + R3) = R3 / 20MΩ
C
C
Current
Limit
S
R
R
Q
85 nS Delay
Voltage
Doubler
HV9606
PGND
SGND
GATE
VX2
Vdd
CA
CB
CS